Marquis isn't amazing or anything, but he can guard three positions, knows the Celtics system well, and also run the point in a pinch, so he's a hell of a utility belt to have on the team. I wouldn't take one of the rookies over 'Quis unless we first replaced what he brings to the team.
EDIT: why are there about a bajillion people viewing this thread? Is the fate of Keyon Dooling so interesting to 200+ people?