Disc + Renew = True?

Don't use renew if you're discipline.

That is what I've been hearing since forever, heck what I myself have been proclaiming for all of Cataclysm (and Wrath). It's been a truth for us for so long that we've probably completely missed out that it might not be true anymore. I remember when Renew was so bad I in fact considered not even using it in my holy spec. It was too weak and too slow to be worth the time and effort, and as holy I've already got a bunch of nice instants and on the run casts, so why waste a gcd on Renew? This was even more true as discipline since I lacked most of the talents that really could've made Renew worthwhile. But things have changed, and I think we need to take a second look at Renew and how useful it is to us now.

Renew has bad hpm
This definitely used to be true for a discipline priests, and is probably the main reason we decided that Renew was the enemy in the first place. But does this argument still hold? If we compare Renew to some commonly used spells;

IF = Inner Fire
IW = Inner Will
Green = Disc
Pink = Holy

we'll see that the hpm of Renew is far from awful anymore, in fact the difference in Renew hpm between Holy and Disc is about the same as often used spells like Heal, Flash Heal and Greater Heal. When disc it actually has a better hpm than Greater Heal and equals out to Heal, if we use Inner Will instead it has better hpm than Heal. Eventhough hpm always is subject to gear and other outside factors, this will hold true for most of you, and in any case it definitely warrants you to take a look at the current state of your own spells hpm. To say we shouldn't use Renew because the hpm is too bad as compared to Holy, or compared to other commonly used spells means we shouldn't use spells like Heal, Flash Heal and Greater Heal with the same line of reasoning.

It's mostly overheal
As with any other spell in your arsenal you have to decide whether this is true for you or not. Whether you end up using Renew at a good or bad time is nothing I can do anything about. Overhealing is indeed one of the weaknesses of Renew and one of the weaknesses of hots overall, mostly because of how fights (and incoming damage) are designed. As a druid/holy you take the overhealing into consideration when you use your spells, and we would have to do that too. But that is a problem inherent of the spell, not of a disc priest using it. It's not like it magically turns into more overhealing when we use it as compared to a holy priest - what we really have to focus on is whether it is worth the mana and gcd. We've already established that it's worth the mana, but what about the gcd?

It doesn't heal enough
It's a fact that Renew is weaker for a disc priest than a holy priest, and part of the long reasoning behind not using it at all has been that it hasn't been worth the gcd. Because even if the hpm is good, the hps might not be. I do think that out of all the arguments against using Renew, this probably holds the strongest. I'd even say that in most cases you'll probably want to use another spell, so what is Renew good for? First of all we have to remember that a lot of the strength from Renew as holy comes from talents and glyphs that are available to us as disc if we'd like. I'll admit that what we have to sacrifice to get Improved Renew probably isn't worth it, but Glyph of Renew might very well be. However, the real strength of Renew is that it also fills one big spot in our healing arsenal that currently is only filled by two spells, both with relatively long cooldowns. Just as with Holy, we should consider using Renew as soon as we have to move around, which in current content is quite often. Right now we've got the Shield and Prayer of Mending (or of course, Holy Nova), but both of these options are soon exhausted and on some fights with a lot of moveement there might be a lot of downtime for a disc priest unless we want to spam shields everywhere (tornadoes during Alysrazor for example). Instead of thinking that the Shield is our only option we should definitely consider spamming out Renews on anyone who's taken some damage - it is basically as throwing a Heal or Greater Heal on that target. The Shield isn't a heal in the first place, and we have to be able to acknowledge the times when the Shield won't do any good. During those times a Renew might very well be the answer. Renew isn't a substitute for the Shield, but a complement and should be treated and used as such.

And also
Renew does benefit a lot from haste being a really good stat choice for discipline priests. Stacking haste like I have ensures we easily get that extra tick. Each tick procs a trinket like DMC Maelstrom, can crit and crits procs Divine Aegis - that's worth noting too. If we know how and when to use our Renew it really only has one major drawback, and that is that it doesn't scale as well with spellpower as most of our other spells. Nonetheless, even without proper talents and glyphs it holds equal footing to spells like Heal and Greater Heal as shown above.

Other people have also voiced their interest in the efficiency of Renew, and whether it is worth implementing into our healing rotation or not. Here are a couple of other things said regarding the usefulness of Renew in t11 from forum threads here and here;

Venaliter;

"Renew is just prone to overheal, but it has it's uses. It also has its detractors. In a way, they are right. You really should be casting penance, prayer of healing or mending and Shield. (higher hpm/hps) and renew takes a backseat to all of those.

  However, I use Renew for things like furious roar, to stack healing on the tank while I am CC'd. If you know a big hit is coming, it's good to cast Renew on the tank pre-empetively."

Evrion;

"I love renew in PvP, great for folks taking a little damage or with DoT's on them that I don't have time to dispel at that moment. At least slows the drain until I can cast something larger."

Khendra;

"If you're willing to invest in Renew it can be very good.  Renew hits 5 heals at 12.5% haste, which is easily reachable with nominal rating (~5%) and raid buffs, or automatic after 2/2 Borrowed Time.  If you keep a constant 12.5% haste then you get 6 heals any time you have BT + Power Infusion/Bloodlust/Time Warp/Ancient Hysteria.  5-tic Renew is extremely efficient, talented and glyphed healing for nearly as much as Greater Heal for 1/3 the cost.  6-tic Renew is the most efficient heal we have available. 

Mechanically, your raid should not be topped off all the time, making Renew an ideal tool for keeping people out of the danger zone.  Renew is also the only means we have of casting two heals at once, effectively stacking throughput, which is very handy on tanks.  Renew also helps proc chances on most trinkets and all weapon imbues.  It's instant-cast and good for periods when your raid must move, extremely handy when used in conjunction with PW:S. 

In short, yes, Renew can be good for any flavor priest."

Safia;

"Is renew good for disc?  The answer is pretty simple to me.  It depends on what you are doing and who you are doing it with.  25 man hardmodes, I wouldnt touch renew with a 10 foot pole.  10 man regular mode, depends on your healing assignements and who you are healing with.  5 man heriocs, I use it all the time."

And a thought from t12, taken from a mmo-champion thread.

Peppercat;

"Here's the truth about renew for disc; it's situational and only to be used if the following are true:

1.) You're moving

2.) PW:S is on the target/has been used on the target/You have put the amount of PW:S your gear can handle maintaining on the raid

3.) PoM is Out

4.)You are NOT moving into a stacked pile during an AoE damage mechanic where Holy Nova till Stationary is better

If these are the circumstances, renew's niche becomes clear. It's also not terrible to keep rolling on the tank, but definitely not needed.

Renew has it's place in the disc arsenal, and anyone who tells you otherwise is spewing bullcrap. "

Overall priests nowadays mostly seem to argue whether it is needed or not (kind of like the discussion about Atonement/Archangel), not whether it is worth the mana or time or not. Only you can know that for sure really. The above four points are pretty good ones (although I don't completely agree about the Holy Nova one since HN has a cap and really heals for so little).

The short version of it is;

Should we use a gcd for Renew instead of any other heal? No
Should we use a gcd for Renew when there is no other heal to cast? Yes

Like I said earlier it's not supposed to substitute for our other heals, and if you're standing in a safe spot you should probably always use another heal over Renew. But it is still a fact that our on the move healing lacks something and we shouldn't consider Renew completely worthless. As such, Renew definitely has a spot in our healing arsenal.