i love my mixed team, people can say that intellect is a useless buff and that mages have poor mana regen but if you plan to raid, you will want the intellect buff. teleports are very nice and water is also very nice. some people are happy to buy tea to replenish mana and that is fine, but mages bring a lot to the table and as arcane my mage can top 4k dps often. yes he has mana issues but i have a key bound to use mana gem and one to use evocation. basically i bound the "use mana gem" key to also cast thunderstorm for the shaman and to the mana regen on my shadow priest as well.

i'm currently leveling up a lock to replace my prot/holy pali for times when the raid doesn't need 75% of a healer or an offtank, but for heroics you'll obviously need your own tank.

play what sounds fun to you, for me i wanted to maximize not only the buffs i cast on my group, but also the debuffs i could put on bosses.