>but then there's thermal throttling as well. You can't run all of the logic units flat out on a bunch of models of CPU.

That doesn't make any sense. Disabling SMT likely saves negligible amount of power, but disables any performance to be gained from the other thread. If there's thermal budget available, it's better to spend it by shoving more work onto the second thread than to leave it disabled. If anything, due to voltage/frequency curves, it might even be better to run your CPU at lower clocks but with SMT enabled to make up for it (assuming it's amenable to your workloads), than it is to run with SMT disabled.