Supporting Trump IS acting on principle. It advances the cause of conservatism...if nothing else than by stopping his far more Left wing opponent. It also ensures the Supreme Court does not shift radically to the left FOR A GENERATION should Hillary win. There are only two realistic options...that's it. And when you really do only have two options, picking the lesser evil IS the principled thing to do.
I agree with your point, although there is no way to know whether Rubio is actually doing this for the reasons you describe, or because it thinks it will help him politically. I tend to think it's both, which is fine. But I can also see how someone could come to a principled conclusion that electing Trump will not advance the cause of conservatism.
In other words, I think there can be principled people on both sides, and we shouldn't be assuming the worst about others simply because they reach a different conclusion. That's different from arguing about specific issues relating to Trump, though.