Because tanking is harder and if healers realized that they would stop whining? NO. Running some instances as a tank, getting ok at tanking, will make you a better healer, and here’s why.
You’ll know what the tank is going to do next, and why. If you’ve been healing for a while you probably already have instincts that tell you when a tank is going to LOS a pull or charge into the next one, but once you’ve tanked those pulls yourself you’ll know why. There is no substitute for actually doing it.
You’ll know what a tank is capable of, and what he’s not: This will make you more tolerant when he loses aggro, remind you to keep an eye on those dps-ers pulling 5k damage, and know when to have your emergency macros on hand.
A change of perspective helps: I’ve run Northrend dungeons dozens and dozens of times on my mage and my tree, but it wasn’t til I tanked them on my paladin that I realized certain things, like that some of the portals in VH always spawn groups while some always spawn a single mob. Or that those stupid orbs on the platforms in OK do LOS. Or that there’s an invisible wall in TOC by the door the enemies come in that they can get through but you can’t, so you can’t hit them.
You learn what healers do that makes tanking harder: I often start dropping HOTs on the tank as he pulls. Turns out it can be a bit annoying when the healer instantly pulls off you when you’re tanking. Makes life tricky. I’ve started looking for other strategies.
You learn just why tanks are always wanting to go-go-go. Oh, rage really is a pain to regain once it’s gone. And Divine Plea needs to stay up, so either start the next fight in less than twelve seconds, or wait out the cooldown til it’s ready again. And the mobs move around, so waiting ten more seconds changes the battle space. Not that any of these concerns match “healer has no mana”, but maybe if I’m in all tier 9.5 or better and have 70% mana I can handle another trash fight…. yeah, probably.
Because there’s nothing you can’t do: We all dps, for daily quests or leveling. You know deep down you might not be as good as the rogue who tops the damage meters, but you can do it. But tanking seems like an esoteric skill. It is, but no more so than playing whack-a-mole with 25 health bars WHILE staying out of the fire. Give it a try. Most healers will probably find that they still like healing best – I know I do; I think raid healing 10 mans is my favorite part of the game by far. But I can tank, and I can tank pretty well, and just knowing that is empowering.