Glen Davis and Rasheed Wallace are both below average defensive power forwards and neither player can adequately defend the pick and roll against a Wade + a jump shooting big man (Beasley/Haslem).
Rasheed is more than an average defensively. Are you saying that he's above average as a Center? and not as good at the 4?
He's well below average at either position at this stage of his career. Feet don't move!
I'm sorry, but that is not true. He is probably only an average defender at the PF spot nowadays (due to lack of quickness), and also a mediocre help-defender, screen-roll defender, perimeter-defender and maybe even team-defender in general.
But he is still one of the top 5 post defenders in the league! Did you see him in the games against Orlando? Dwight couldn't do anything against him. Sheed was defending D12 better than Perkins did in those games. He has the perfect mix of strength, raw bulk and height to deny position. He has got the guile to throw off an opponent who is backing him down, and his hands are so quick he deflects and steals so many dribbles and entry passes.
He may not be an amazing defender overall (as he once was), but he is still one of the best defenders at the center position, and one of the few who can handle Dwight Howard (Yao, Bogut, Perkins, Duncan, KG, Rasho, Sheed, Shaq, Kurt Thomas, Pau Gasol - not in order). Don't sell him short.