There have been times where I've thought maybe...but ultimately I have to say no.
It's not because of any of this 'traitor' garbage people are talking about either. That honesltly means nothing at the end of the day, because all you should care about is whether the player makes you better and increased your chance of winning a title.
At this stage I have to say no, he doesn't. Ray and Pierce IMHO have taken very similar paths the last two seasons. Both of them simply know how to score and how to shoot, and there's no denying that. However, both of them seem to be at the point where their bodies can no longer handle all of the things that they, mentally, still feel they can do. They can both still be deadly, but only in stretches - these days they are both struggling to shoot the ball consistently.
The difference though is that Pierce was never super athletic, and his advantages were always his size, his strength, and his offensive IQ. He's a step slower but he's still big enough, strong enough and smart enough to be productive offensively and defensively.
Also unlike Allen, Pierce continues to be an outstanding passer and rebounder, so even if he's not scoring he can still contribute. Allen's shooting / scoring is really all he has to offer, so when he's not scoring he's a liability on the court.
As average a season as Terry had he doesn't look anywhere near as old, rusty and washed up as Allen does. He still looks well conditioned and still has decent atleticism, he just seemed like he struggled to fit find a clear role. When he's hot Terry can be as deadly as Ray can (as he showed a few times in the playoffs) and unlike Ray he has the ability to play multiple positions which IMO is a huge benefit.
Also Ray is older, and you are looking at a celtics team that is trying to tread the fine line of getting younger (for the future) while still building a roster that can complete for a title. Adding Ray makes us older, when we really want to be getting younger.
If he never left, then I'd have loved to see Ray here if only to see him retire a Celtic along with Pierce and KG, and finish up the entire era together. Ray already broke that sentimental reason when he left, and he just doesn't have the talent anymore to bring him back on ability alone.