http://en.wikipedia.org/wiki/Dragon_Age:_Origins <----- has your suggested system implemented word for word. Activating buffs cuts into your total mana pool, deactivating raises the cap again.
I personally didn't like that system. It did fix those problems, but it also meant that you had two choose between passive, always-on abilities and using cool actives. Which meant that casters could becomes static buff machines with poor ranged attacks or actually contribute with tactical choices in the fight. Or a combination, but with the generally low pools of Origins, that was pretty bad. Unless you utilized the Alchemy skill and had an eternal amount of potions thusly making the capped pools null and void. Maybe instead have two pools; one for buffs so you have to decide
between buffs, and one for actives. I dunno. I liked the attempt DA:O made, I just didn't like the choice between passive and active abilities.
Or just have buff "slots" or something like that.
EDIT: Wait, aren't you one of the guys in the P:E thread always raging about DA:O being like an MMO? Pretty dumb if you don't even know its basic systems. Forgive me if I mistook you for someone else.