PvE: If the boss hits the tank for 2 damage every turn that's predictable and you can play either around that or along that, saving a lot of mana in the process. And given the right/wrong expansion that can be an important factor. If the boss hits your tank for so much that it's close to alive:death switching status you must start pumping heals into him no matter what - while stopcasting might help in that situation it depends on the boss swing timer and other fight mechanics. Ever bothered with patchwerk? That's a perfect example for this. Or Baleroc during decimation blade.
PvP: if i can bring you to 50% health with my toothpick, pop everything and you die in the next two globals that's not something everyone can play around at all times and that's why several classes/speccs are dreaded.
Consistent, predictable damage almost never kills you and your team in any situation, it's when something happens that you don't have control of (take Spoils when your healer gets the bomb back to back and you lack hybrids).
You might want to raise the "but i don't pvp, i don't care" flag but i'd call you ignorant. Having burst is just as important for raiding as it is for all other occassions. You can rest between waves of enemies, but you'll have to kill them ASAP when they spawn so they don't overwhelm you with their magic. That's not an unlikeable situation and argueable already implemented in boths dungeons and raids (ie: bladestorm on Garrosh mindcontrol).
So if they'd design our specc about ms, autoattack, deep wounds, slam and colossus all hitting for 1 and execute hitting for 9 it would probably be balanced in a nutshell but it's unhealable in a certain environments.