The problem with not having one of them be healing specced is that the game is now balanced for every main healer having at least one heal which will heal up multiple people at once. I can't think of a tank and spank boss in WOTLK which doesn't also do something that damages the whole group, and in some cases significantly so. Like Moorabi, he's so easy with a priest AOE healing the quakes, but those quakes will do a good 10k+ damage to the whole group by the end of the fight WHILE he's still hitting the tank, and that's pretty much par for the course now. If you had tree form you could just hit one key and get a HOT on everyone at once and be fine. The single target way won't be really quick enough to be effective, especially with non-healing specced characters.