Some over at Gamefaqs despise him because he doesn't fight.
The fact that he doesn't fight is actually what I think is cool about him! He's unique; he uses his intelligence rather than powers. He thinks out the plans and has Lexaeus and Vexen carry them out. HIS methods (non-violence, dark powers, mind games) nearly killed Riku, while Lexaeus' methods (Pure violence) failed DISMALLY.
Give Zexion's methods credit, because if it wasn't for Namine, they would have worked.