Hybrid classes are win.

It's convenient to have 1 account with tanks, 1 with healers.

Just think about it like this: if you use 1 out of the 3 chars you boosted, you have not wasted time. Personally i only leveled my shaman to 60 in my RAF and i regret it. a lot.

With shamans/paladins/druid/lock(+priest) you got a lot of possible combo's.
For pve my current team is protadin, disc priest, ele sham, fire mage and destro lock and loving that. Multiple toons is definitely a lot of fun to play for pve. So that's a personal thing to think about.