Enforcing strict rules on holy Healing causes the correspondent rules on how people should roleplay their injuries. Healing trough holy means is a grace to roleplay, in a way it is much like deus-ex machina (even tough it is backed up by lore). Of course, risks, consequences and failings of holy magic are to be expected, but seeing it as a bad thing or an escape for lousy roleplayers is the wrong way to look at it.
Instead the roleplay that actually happens should be the matter of things. A priest who has devoted his whole life to his holy grace and never swayed from his lawful good alignment is the right way to do it. Hell, I wouldn't even mind if a nun simply snaps her fingers and some holy light replaces a broken bone.
With g.o.o.d roleplay come g.o.o.d deeds. Looking deeper into it, I think the problem lies in the whole tiermaxing crap that many people pull off. tl|dr:
Remove tiers from holy magic, instead focus on alignments