I can forgive arcane casters who don't blast, fighters who don't shield or rogues who don't disarm traps, but clerics who won't heal are something else entirely. I end up playing the cleric a lot and when I do, I heal. It's what I'm there for. Clerics who don't heal, or who charge party members for healing, do not understand the niche they're filling and have no business holding all the healing cards.
I don't think roleplaying reasons, using the paladin and the thief as an example, are a valid excuse most of the time. By inaction the healer is willing the other character to die. By your character's inaction you, as the player with the healing monopoly, are willing the other player's character to die. This paladin didn't want to risk a friendship with the thief by coming out and accusing them, but was okay with leaving the character to die? That's pretty messed up.