I often get to hear ideas for role-swapping in WoW and I generally encourage most kinds of experimenting in a WoW setting. That's one of the reasons I've written a post about warlock-tanking and expect to write more in that matter. There are however times when role-swapping isn't as good, usually in raids. The most recent idea I heard made me think about this phenomena overall and so I thought I'd write some of my thoughts on the matter.
This idea I heard about wasn't bad per se. Some guy had thought about speccing full Greater Heal to maybe turn a priest into an awesome tank-healer. Yes I've had this thought too. I've even gone so far as to actually try it out in a raid, and it does indeed work fairly well. So hey, let's go crazy with our specs! No, there is something that needs to be put into the calculation here. You are not the only healer in a raid (or at least you'll very rarely be).
The other healers in the raid have their roles to fill as well, and optimally you will complement their roles with your own. That way you won't have only healers who are good at say healing the raid and not tanks, or vice versa who are good at healing the tank but not the raid. Some classes don't have much options, paladins will for example never be good raid healers. Ok ok, wait a sec, maybe we have to take another step back at this and start at the way beginning.
Any healer can deal with most kinds of healing that occur in ICC, so when I say that a paladin won't be a good raid healer I don't mean they can't do it at all. Fortunately there are no bosses in ICC that heavily penalizes one type of healer, like Shirrak in AC. But ask anyone playing a certain class and they will tell you that they can handle one type of healing better than another. The paladin/shaman usually wants to stand still to spam their chain heals/flash of lights/holy lights/etc. The priest and druid is little more ok with running around. The druid generally doesn't like having to spam one target, since many of their spells are well used on many targets, same goes for priests. This doesn't in any way mean a paladin can't raid heal or a druid can't tank heal. They probably could do that pretty well, but their potential would be wasted.
And this is the point I'll stress here - potential. When experimenting with your role you have to take the potential of yourself and other classes around you into consideration. The same goes for some dps cases of course, you wouldn't want to have the rogue use Expose Armor if you've got a prot warrior using Sunder Armor. It is a waste of the rogues potential to have him waste combo points at something the warrior will do better/more easily.
And even though a priest might turn into a pretty good tank healer, there are already healers who do this job really well, but who can't in turn take the role the priest is leaving, that of a raid healer.
There is an old swedish saying that goes "Cobbler, stay at your last" or in modern english "do whatever you're best at". In real life I don't believe people should stay at one thing if they don't enjoy it, but in a game like WoW which basically revolves around numbers this will actually matter. Your and other players chars are only really a bunch of numbers standing against the computers numbers (npc). Therefore there will be an ultimate equation, one that is better than others.
Of course, role-swapping could be a great way to spice things up, like with lowbie-tanking and even in raids if your group think that the fights are way too easy anyway. Just remember that role-swapping has its time and place and that you've chosen your class for a reason - stick to it.