He might be an overall liability, actually, and the Bills might do better letting him go after this season. Look at the last game, for instance. Five targets, one catch for a TD, one drop, one interception that arguably was his fault. Not a fair sample, maybe, but his career catch rate is low, his drop rate high, he's not quick, has a hard time getting open, and often runs lazy routes.
The positives people have pointed out (except for the blocking) flow almost entirely from playing with a future Hall of Fame quarterback.