Well...in terms of healers, we just get lazy in raids I guess. Especially ones that never drop gear for us. I ran Void Ark 10 times in a row to get the robe(still have not gotten it) and I got nothing that I wanted/needed in those 10 times. I bet by the 3rd time I was half asleep....

Also, most raids tend to be unsynced and healers can heal through the AOE's/mechanics a lot of the time.

In general people just stand around also because raids can be boring if you do one too many times or are unsynced and it is very easy.