I had an amusing thought about healing Priests in WoW.
Traditionally, the problem with hybrids is that while they can do multiple things, they can't do them as well as a pure class. You can tank, heal, or DPS, but not as good as a pure tank, a pure healer, or a pure DPS. And when putting together a raid, rather than taking two hybrids, you get better performance by taking 2 different pure classes.
If you look at the healing classes, you can see the same pattern emerging. Paladins are the best single-target healers in the game. Druids are the masters of heal-over-time spells. With Chain Heal, Shamans are number one when it comes to healing splash damage.
Priests can do all these things. They can direct heal, they have a solid HoT, they have decent splash heal tools. But they aren't the best in any one category. They're second-best in all 3.
Priests are the hybrids of healing. And thus they suffer from the traditional problems of a hybrid class.