Winning races helps keep me from burning out. True story.
If one is getting burnt it's a pretty good indicator that something is awry in the training plan.
I find a PM helps me on the high and low end. Low end, I go too hard when I should be going easy. High end, I've got more in the tank than I think. The numbers don't lie.
I can guess tempo range stuff pretty accurately all day long.
Of course, I won a few races without the PM. I've won more, and bigger races with.