In 50 starts this season, Jason Thompson average 12.8 points and 8.6 rebounds in 32.4 minutes. In 12 bench appearances, he has averaged 11.3 points and 7.5 rebounds in 25.9 minutes.
So clearly, J.T. gives you about the same as a starter or reserve. But those 6-1/2 minutes matter -- Thompson is giving you just 1.5 less points and 1.1 fewer rebounds in 6-1/2 fewer minutes. This is why we use per-minute statistics: to give us apple-to-apples comparisons.
If you adjust Thompson's bench performance up to 32.4 minutes, Bench J.T. would produce 14.3 points and 9.4 rebounds. That's improved performance off the bench.
Watching the Milwaukee game (in which Thompson had just 9/3 in 28 minutes), it seemed as though having fewer offensive options on the court improved the flow of the game in which Thompson participated. Playing with Tyreke Evans, Beno Udrih and Spencer Hawes, it almost looked as if Thompson had to shoot every time he received the ball in scoring range -- otherwise, he'd only get three FGAs a game. Off the bench, playing with Francisco Garcia and Ime Udoka and usually Sean May but sometimes Andres Nocioni ... there are shots, there are plays available for Thompson. And this isn't always necessarily a good thing -- he remains the least refined offensively of the rotation bigs. But it seems like it calms Thompson down, like he values the basketball and each possession more when he knows he'll get another chance, that he doesn't have to take the baseline jumper or pull a spin move out of thin air.
This theory could be worth nothing, and even the bench production data could be subject to sample size noise. But it seems like the bench is a better fit for Thompson, and I'm at a loss to find other potential reasons why.
If Thompson can keep it up, that's really great bench production, 11/7+ in 26 minutes. Landry averaged 16/5 off the bench this season in 27 minutes. For his career the excellent Udonis Haslem is 8/7 in 24 minutes off the bench. J.T. serves as a middle path of sorts -- a good rebounder like Haslem, a good scorer like Landry (though not as good at either as the respective skills). Thompson could end up as a really good third big man, whether Landry/Hawes works out or whether the Kings go and get another big man to add.