I think Cullen would be a fine addition. I mean, sure, you have Alistair who is a Templar, but it would be nice to have Cullen along to. He wouldn't be sword in board, as it looks like he uses a very large two-hander sword (sure, you have Oghren and Sten, but couldn't you just pass up Sten?). Besides, with Cullen on board, you could focus on maxxing his Templar abilities while putting Alistair's to use in his Warrior/Sword and Sheild trees and maxxing them out.
Besides, with a Mage, it would be a great thing to have and I would love to see if you couldn't make him renounce his vows as a Templar and have a relationship with him. There's just too much character development for him to go to waste like that.
And if you think about it, it's fair. If you side with Templars, you get screwed out of a party member, because you have to kill Wynne, the most handy healer in the game. If you spec yourself healer as Mage, then side Templars, you should be able to at least make up the difference with the extra party member. That way, you've got extra damage while you stand back and throw heals.