As a healer I've always had a very special relationship to spirit. It's been with me through good and bad, to some of us it has been among the best and most important stats while others have just shrugged at it and chosen intellect instead. It made gearing for a disc/holy priest like me living hell in Wrath, since it was a great stat for holy and not so awesome for disc. To holy it used to give both spellpower and regen, something Blizzard removed with Cata (giving us pure regen instead). This is really taking a step back to how Spirit first used to work for us healers.
In Vanilla most healers focused on something called the 5-second rule. Since our main way to regain mana during a fight was done when not casting, we had to find good opportunities to pause our casting to regen mana. This meant careful planning of our heals, and was nothing like the heal-spamming that was to come in Wrath. There were exceptions of course, paladins could often heal both spammily (yes, that is a word now) and endlessly thanks to the fact that all ranks of heals (back when we still had ranks) were equally affected by spellpower. This way paladins could use rank 1 Holy Light or Flash of Light, which cost nearly no mana, but still healed loads thanks to a bunch of spellpower. Other classes, like mages, had a certain gear set for equipping only when they used Evocation, only to get as much out of it as possible (back when you could swap any gear mid combat and Evocation was based on spirit). Back then Blizzard came to the conclusion that to reward healers for not healing seemed a little counter intuitive (although they seem to have wanted to return to that somewhat in Cata). Instead they wanted us to base our choices not on when to heal, but who and how. Healing changed quite a lot with Burning Crusade and although the 5 second rule existed all the way to patch 4.0.1 when it was removed, in practice it hasn't been in use much since Vanilla.
The removal of the 5-second rule with Cata means that spirit in itself doesn't give us any mana as long as we are in combat. But Blizzard had thought of tools that would help us with our casting/combat regen already before they made this change. Back in BC, because spirit would only affect regen out of casting, Blizzard implemented two "tools" that would make mana less of a concern for healers. The first was mp5, which they also removed with Cata to return healing more to the state it was in in Vanilla (and also because they did a general streamlining of stats). The second was various talents that would allow for our spirit regen to continue while in combat. These talents can look differently, but they have the same result - more mana. Holy priests for instance have Holy Concentration, which in its current shape allows for 30% of our spirit regen (that is out of combat mana regen) to continue while we are in combat. Another example is Clearcast, which by giving the healer a free of mana cast allows for him to stay out of casting for a longer period of time, making the 5-second rule stay in effect longer. Before they removed the 5-second rule, CC worked a lot like any regen talent. Because of the removal of the 5 second rule, CC has been nerfed since it now only gives a free cast, and no extra regen (since there is no out of cast regen any longer). I am not sure if Blizzard compensated for this by making CC proc more often, but it doesn't seem to be a major concern for druids in any case.
Like mentioned, spirit had a prominent role for some healers in Wrath, providing with extra stats like spellpower, while others had more gain from Intellect. In BC Blizzard also connected spirit and intellect by making spirit regen more the more intellect you had. Intellect became more and more important since it increased our mana pools, gave more crit and then also the added regen to spirit. Skills like Replenishment and Rapture were affected by max mana, not spirit, which made intellect even more important. The only healers who decided to stick with spirit were the ones who had some extra benefit from it. In a sense Blizzard had divided the healers into the intellect-regeners and the spirit-regeners, but overall spirit was definitely going out of style.
In its current implementation, what makes spirit so interesting is that it is both one of our most important, and least important stats. In itself, it doesn't actually really do anything for our healing. It doesn't actively increase healing output or throughput. It only makes sure we can heal at all, which in a way is the most important factor to any healer. Spirit could be compared to the dps stat hit - it doesn't do anything in itself, it just passively makes sure that all the other stats (strength, agi, crit, haste) are worth something at all. If you don't hit your target, your crit isn't worth anything. Spirit works much in the same way by providing us with more mana. If you don't have mana for your heals, all the haste in the world won't do you any good. But just as dpsers make sure to only get a certain amount of hit, I would like to argue that we healers should only get a certain amount of spirit. Some healers stack spirit, but I just don't see the point. Because spirit isn't worth anything as long as you actually have mana, there is no use to it whatsoever after the point where you have enough mana to tackle an encounter. Spirit works a lot like it did for those evocating mages back in Vanilla. As soon as they were done getting their mana, they quickly removed all their spirit again. Unfortunately we don't have the option to swap gear mid combat, but we can still keep spirit down to bare minimum. We should only use it as long as it is beneficial, and go for anything else when it isn't. The key is of course to identify that limit.
Unfortunately, that is far from as easy as just reaching some certain percentage as with hit. How much mana you need to have all the mana you'd might want during any of the current raid fights (and/or heroics) depends on a lot of different things. Not only what class and spec you happen to be, but also what the rest of your group looks like, and as mentioned your mana pool has to be able to sustain all the current content that you're interested in running. Change one of these elements and it is back to the drawing board. I do understand it if healers just decide to go for as much spirit as possible to cover all bases - on the other hand this is an endless resource of stat-tinkering if you're into that sort of thing. Back in Wrath when only a few classes had replenishment skills, you really noticed the difference in your mana pool when you didn't bring one of those classes. Nowadays we notice that druid innervates and to an even greater extent, shaman mana tide totems make a whole lot of difference. Whenever I oom on a fight where I usually don't, it's always a sure bet that we brought less or no resto shamans with us.
But it's not an impossibe task. Many of us, myself included, don't run in groups that change that much. Few raiders today can count themselves as lucky enough to have an active raider base that allows for major changes in a group setup. Changes do happen of course, and when we had one resto shaman quit and the other one go on a minor break, I noticed at once on my strained mana pool. I have gemmed and reforged with those resto shamans in mind, and removing them from my regen calculation made a big difference. Loads of spirit is also useful if you're prone to dying and being resurrected, because you've suddenly gone to 10% mana a lot faster than you had anticipated. Personally I don't believe in making too many changes to accomodate the "dying scenario". Hopefully any changes made beforehand will prevent you from ever getting into that situation. For heroics it's probably best to not count on any sort of support regen. There won't be a resto shaman there with mana tide, unless you happen to be that shaman. Don't count on the druids using their innervates on you. The chance that you'll be completely without replenishment is small, but still there. Point is, in most cases you'll be able to estimate your personal needs for mana pretty accurately if you just put some time into it. Once you have that figure you should add some to give you a little leeway, and then convert any excess spirit into more useful stats like mastery, haste or crit, depending on what happens to be your cup of tea. There are tools that can help you with these estimations, like any combat logger, with which you can get raw numbers of your mana output, compare between fights and get a final number on what you need to tackle the fights you're currently in. Another great tool is good old experience. Once you get to the point where you've done the same fight a couple of times, you're probably into a certain "rotation" on how you deal with the fight and should know fairly accurate how much mana you put to use. You're probably put to do the same task in each fight respectively, so even if some elements of the fight will change, you'll still have a pretty good picture of how you heal the fight. As soon as you notice that you have way more mana in the end of the fight than you should, you can start looking at converting spirit into other stats.
The challenge is to get as close to zero mana each fight, after having burned through any regen tools that you might have (Divine Plea, Innervates, Shadowfiend etc), with the pot as a saver for special occasions. This isn't to put extra strain on your fellow healers, you're not supposed to just stop healing. Quite the opposite, by converting your spirit into more useful stats, you'll hopefully be an even bigger contributor to the overall healing output. Any mana regened at the end of the fight that isn't put to use isn't wasted mana, it's wasted stats. You could've had more haste, crit or mastery for those manas! I make it sound easy of course, but it is far from. Like I said, it is possible, and if you ever find yourself without something to do, you've just got yourself a challenge. Lose those spirits!