First of all, we plan to allow most of the Shaman and Paladin buff effects to stack, with a few exceptions of course. This means that both will be extremely useful in regards to improving the potential power level of those around them. Beyond this, you're only thinking in current “WoW” terms and seem to only be digesting current end-game. When the Burning Crusade releases all classes will have access to new spells, talents and abilities, which will be taken advantage of (and play a large part in) grouping/raiding in the expansion's end-game.

It's also important to note that moving forward, we'll be able to design our raid and dungeon encounters to specifically draw out the strengths of each of these classes, whereas now we actually take measures to prevent an encounter from being too beneficial to only one of the classes.

I'd also like to mention that beyond raiding, we think it will be exciting for players that feel strong allegiance to one specific faction to have access to a class that has seemed unavailable to them