I know that Barnes isn't THAT good a player any more, but in the even of an injury to Pierce or Green I'd feel a lot better with him in the game than Kris Joseph. He's a bigger small forward, too, so if any injury to Pierce/Green were to occur, you'd have the option of either going big at the SF (Barnes) or small (Courtney Lee). I like that flexibility.
As to Barnes being an annoying ****... well, so's the rest of our team. Once he's on our side, who cares?