I don't beleive groups really organize themselves around healing niches in the sense that they want one hot guy, one AE guy and two direct heal guys. We just want enough distinction that you consider bringing e.g. a druid as your third healer once you have two priests.
I'm really surprised by this. Ok, its pretty easy atm . But I've always invited healers in a raid by healing niches, according to what we need to suceed.
Druids for hots, pallies for MT healing and their bacon of light, shammies for group heals and priest for group heals and whatever needs being done. Yes there is some room for flexibility - but I'm really surprise by GC's comment.