My concerns are that he looks for his own shot a bit too much (even at the expense of better players), and his defense is quite poor.
I don't think those are concerns. I think those are facts.

Thomas isn't ever going to be a great player. But he is a young, good player on a team friendly contract. And to get that kind of player for what Danny just sent over is pretty remarkable.
Moreover, he should really fill a need. The Celtics need someone who can score off the pick-and-roll, who can create a shot and who can fill a starting role. Thomas can do these things. His biggest deficiency is his defense and the Celtics have two of the better guard defenders in the league in Bradley and Smart.
Finally, if he doesn't work out, what did the Celtics really give up? An expiring contract and a very late first (something that we could easily get again by packaging a couple of the 2nd rounders together). This is almost a no-risk, potentially high reward deal that helps the team now and in the future.