I used to be full holy spec while I was still running instances, and believe me when I say that the holy tree helps a LOT more than you think as opposed to speccing shadow or disc then being a main healer.
The issue is that priests aren't FUN where they're most needed. Everyone wants you to group with them, but they expect you to just healbot all day and you end up watching health meters the whole time. If something goes wrong, the first thing a lot of people say is "where were my heals" when the fact is that the group was simply outmatched or unprepared. I don't even run instances or raids anymore, it's all about the battlegrounds. In the chaos of AV no one gets upset at one person if they die. I'll heal if I notice you're low, I'll nuke if I see a half dead horde trying to run away and I'll dot, fear, and dispel liberally. It's so much more enjoyable than keeping noobs alive, and I can't see how a talent review will change that.