also the answer is due to the fact that tank spike dmg is really higher than anything found in naxx or ulduar now. If i could relate any of it, its like patchwerk 25man with tanks in heroic blue gear sometimes.
ToC is fine with all healing spells. heroic "ToGC" is where the spamming returns and it continues well into ICC. Right now its just the nature of the games as paladins are the only healers capable enough to match the dmg bosses are causing to tanks right now as far as raw healing goes.
Therefore, the majority of holy pally players are obsessed with the notion of infinite mana pools and strive to have the most...which is a bit retarded since having so much int means you have no mp5 and that leaves you 100% depended on crits.
oh yeah, good example of what i mean: Festergut is a newly released boss who is a perfect test to see if your healers are pro and if your dps is worth keeping in the guild. During the fight he will start to rape the second tank (threat switch off) and it is 100% dependent on a paladin to heal it with HL spams until the 3rd spore spawns or its a wipe.