Pretending to lose can be a good move, and if you are able to play it at the right moment, it makes you stronger.

Did Quirrell ask Harry to accept some unrepairable damage? No. It was only about signalling, and temporary pain (any resulting damage is guaranteed to be healed magically later). Quirrell taught Harry that signalling defeat is not the same thing as being defeated. Just like Voldemort, pretending to be killed by a baby, is not really dead.

(I agree that asking for a worthy adversary is suicidal. Having a sparring partner can be useful, but you should be able to destroy them reliably, when necessary.)

EDIT: Though, you have a good point. Willingness to simulate defeat may reduce emotional barriers against (real) defeat, which in some circumstances could weaken one's resolution to fight. Humans are not perfectly logical; when we do something "as if", it influences our "real" behavior too. That's the essence of "fake it till you make it" self-improvement... or perhaps, in this specific situation, self-weakening.