Yeah, I suppose, but since Cyrodiil didn't have that, and since being an actual member of the Mages Guild would probably be incredibly boring for the majority of people, I think they made the right decision. Who wants to sit inside a guild all day studying magic when you could go out and fight necromancers, a small undead army, and the King of Worms?
Well, the Mages Guild quest line in Morrowind wasn't boring, but it required you to actually be a mage, and to be proficient in several different magic skills. The Oblivion Mages Guild quest line is really one of the weakest and most unconvincing parts of the game, since one can conceivably bull one's way through it, and become Archmage, without having any magic abillity at all. It's quite stupid, and it's one of the prime examples that people give when they call Oblivion "dumbed down" from the previous ES games.
There should be actual skill requirements tied to guild advancements, as in the previous ES games. Bethesda lost sight of what a "guild" is, with Oblivion.