Healers should have to pump extra heals while being extra careful with mana compared to running with the proper amount, and failing to do this should result in a very annoying wipe. That's a good thing, that's why fights are designed to have 6 healers. It would be doable in 5 undergeared, but you'd have to go through hell and back for it.
Same goes for solo tanking a fight. When done it can make enrages a lot easier to make, but your healers and tank (and DPS) should have to play a lot more carefully than by "properly" doing it with 2 tanks, and slight mistakes should result in a wipe.
If we're here to discuss 5.4, well, I could care less.