players with strong inside games have always given big ben fits. DC abused him last year in the playoffs. DC! He is NOT a great 1-1 post defender, but he's an absolute terror team defender and rebounder.
Rasheed is longer and arguably the better post defender. Leaving Big Ben on Collins (who is of no threat whatsoever) allows Ben to stay in position to get rebounds, and allow him to rotate on drives, either by kmart or jefferson.
That's why Wallace and Wallace are such a great fit on the defensive end. They play to each other's strengths and weaknesses perfectly. Ben doesn't have to guard the oppositions best post player, allowing him to worry about help defense and rebounding, and sheed doesn't have to worry about trying to get 10 boards/game.