It's not a question of what...it's a question of who...
As Disc, most of the time, I'm going to be casting on the tank(s), anyway, because I'll get more healing done (and mana return), by targeting those who take damage the most consistently. So, go ahead and assign a pally to tank heals--I can do raid, too. It's like what has been described. There's a lot of, "heal and shield, where and when", instead of "spam the tank, and pray for a break".
And, besides, whom shall I Smite/Heal, the hapless Fury Warrior?
-"Just like a buzzin' fly, I come into your life, I'll float away, like honey in the sun..."--Tim Buckley