Here's a different thought:
Training to complete physical and mental exhaustion can produce negative effects like some members on this board noted.
But if you can target specific muscles to be exhausted so that you move "correctly", without affecting other parts of the body, then I think the old addage has some merit.
For instance, maybe doing Randori for half an hour straight, or in the case of standup, sparring, an hour straight is going to produce negative feedback.
But if you do say Agete (kokyu-ho) again and again until your arm/shoulder muscles burn and give out (without the rest of your body tiriing out) then you should still be mentally focused enough to figure out how to do that same movement more efficiently, now that you can't use your arm and shoulder muscles.
Same really goes with any Tanren(body training) method.
For instance, do Shiko about 50 times and you notice some stuff.
Do it 300 times in a row (for about 20-25 min), certain muscles start to tire out, forcing your body to move in a way that doesn't require the useage of muscles in the way you're normally used to.
There's some old school teachers who would do the Shiko movement 1000+times in a row for practice. Yet at the same time they said that you could not be MENTALLY exhausted when you performed these exercises, otherwise there would be no meaning to the training.