It's pretty simple.
DPS is just a lot more fun to play. So when you que into a BG and see 3 ele shamans, a boomkin and a shadow priest with NO healers, the complaints turn into making someone do something they don't really want to do because they have a class with healing buttons.
You need healers, it just makes it more interesting. Healing a BG is always about winning, rarely about topping kb's or going for insane achievements. This sucks and the trade-off is that they are necessary and you better hope they are geared and skilled, because they can carry a shitty team to victory better than a hunter off winning 1v1's somewhere and whining when they lose.
Healing is a bit OP, but the solution might be to add utility not to just gimp healing. Playing a hybrid class well in a BG is a blast because if you use your healing buttons well, you can turn any fight because people write you off as just another dps. healers could use a bit more dos options if they are going to get gimped in raw output, disc priest style.