healing as prot is not about landing big heals, it's mostly a "cushion". While on the tank, prothealer will be spamming fol much more than a holy one(that would usually use downranked holy light for keeping it's cast time buff up), so the tank, instead of going down to "omg400hp", he goes down much later, and to "gezz1400hp". It's about buying that extra second for the healers to land THEIR big heal.
the other good role for prot healer is to handle minor raid damage, like the occasional aoe or life tap or whatever. in those situations, you also just need small heals.
In both situations crit is unlikely to be much useful.
for prothealing, mp5 >>>>>>>>>>>>> crit on a long fight.(take off a few ">"s for a short fight)
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Quidquid latine dictum sit altum videtur (Anything said in Latin sounds deep)