I think the thing that some people overlook is how irreplaceable Ray Allen is. Now that's not even to say that he's better than Vince Carter (though I think he is). But Ray Allen meshes with Pierce and Garnett in a way that most other big stars couldn't.
Ray Allen altered his game much more than Pierce and Garnett did. When all three were on the court at once, the C's played the two man game with Pierce and Garnett much more than they did plays that involved Ray Allen. A good portion of the time Allen was merely a weakside spot up shooter, waiting for a kickout to come from Pierce or Garnett on the inside. This by no means was what he was used to doing in Seattle and Milwaukee. And it is apparent it was hard for him to adjust to this role psychologically given his intermittent struggles throughout the year. Thankfully Allen is probably the best shooter in the NBA and could adjust; however, given how cold he got sometimes, it's apparent that it was tough for him to adjust to not being involved in nearly every play.
Furthermore, Allen's shooting range made coexisting with Pierce and Garnett all the more doable. Since all three of them had great range, it cleared enough space on the floor for all three to be dangerous. Take away that range from the equation, and you get something like the Olajuwon/Barkley/Pippen Rockets. Good, but not great, since their games were all based on being 15 feet or closer to the basket.
So I'd be very loathe to trade Ray. The way he's adjusted his game and the specific talents he possesses are going to be hard to replace. Moreover, given the way he's playing now, I see no reason why he's going to slow down anytime soon. Most of his scoring is coming off of weakside threes and screen plays, which is exactly how Reggie Miller stayed in the game so long: the times he takes it to the hoop off the dribble (when he could face banging inside) are few and far between. While I wouldn't bet the house on it, I wouldn't be shocked if Allen maintains his high level of play longer than many people expect.