Actually, I agree with pretty much everything sackings384 said. Against true centers like Tyson Chandler, Yao Ming, Bogut, etc. etc., Shareef struggles on defense, you often see those guys getting rebounds over him and his moves in the post are ineffective because he's bothered by their length. He's basically reduced to a lesser Brad -- he sets up 15 feet out and shoots his semi-accurate jump shot. He can sometimes bother those guys with his quickness, but for the most part he's just outmatched, and justifiably so.
Against the "fake" or bad centers (Swift, Blount, Rasho, etc.), he can be a great asset at center because he can guard them, get rebounds and get his offense going.
Shareef just isn't a center, and while he can beat most power forwards inside and do a good job on offense he's going to struggle against guys who are bigger than him. Since there aren't many good centers in the league right now he's a great asset on most nights, but it's not something that would hold up against the Yaos and Shaqs of the world.