As good as Thomas is on offense, he has been just as atrocious on defense. That's the thing.
He has the 3rd best ORPM in the league at 5.94. Harden and Westbrook are the only two ahead of him. However his DRPM is -4.44. That gives him only a 1.5 RPM which is 15th among PGs who average 30+ min.
For a team like Boston who doesn't have anyone to carry the offensive load, yeah you start him. For a team who has others to carry the load, it's perfectly reasonable to have him come off the bench and match him up against bench players who won't be able to exploit him defensively. You know that thing of hiding your weaknesses and playing to your strengths? The game is won on both sides of the ball, but all the recognition, all the accolades, and all the glamour comes from being good at one end of the floor.
This doesn't make IT untalented. He's one of the best offensive players in the league! But he has his warts and those warts are on defense. If you can minimize the negative effect he brings defensively by allowing him to defend bench players for more minutes, it's going to make him more impactful.
Basically it means that Thomas is who we thought he is, only more so. He's averaging a career high 20 FGA per game this season and his shooting percentages are up about 2% pts from his career averages. He wanted to go to a team that wanted a scoring point guard and Boston has let him be a scoring point guard. I believe the plan for him if he stayed in Sacramento was to have him come off the bench but play most of the 4th quarter like Ben Gordon back in the day. He probably wouldn't be getting 20 shots per game in that role, but he could easily be a 20 ppg dynamic sixth man. The concern was that he would be exploited defensively so you would want a more defensive oriented PG to start the first and third quarters and then you would bring him in when our primary scorers needed a rest and he would become the primary scorer.
I don't think it was a bad plan, but I also don't know that he would have been happy in that role. That front office let Tyreke walk so it probably shouldn't have come as a surprise when they let Thomas walk too. They just didn't know what they had in either player. If there's still a question over whether Cousins and Thomas could co-exist, of course they could. Both players are talented enough to make it work. I don't know that Thomas reaches this same level of stardom staying in Sacramento -- actually I do know. ESPN is based in New England, he would be an afterthought at best on the Kings even at this level of production but in Boston he may be the front-runner for MVP (you'll see, East Coast based columnists will twist themselves into knots justifying it). So given the circumstances, he's in the best possible place he could be and we need to continue looking for a way to win with our present reality. That may start with finding our own dynamic wing player to take some of the pressure off DeMarcus and exploit the very guard friendly ruleset currently in place.