well, if you can get them all 'free' with gifted levels, i dont see a reason not to. I mean, with 5 u can do more damage, solo instances easier, do arenas, etc etc. I am leveling 5 right now, and will be gifting levels to a paladin for a tank for later on, but i plan on keeping em as 5 shaman for the immediate future, to see how far i get with em.

I tried the multiple class thing, and it's a separate type of challenge. Shamans have great utility, but have a lot of totems to manage, so for some, having a dedicated tank/healer can simplify a lot, but it gives you fewer options.