Relentless self-improvement.

In other words, learn and train, and then test yourself in the real world, in a situation where it's possible to lose.

It seems to me that doesn't fall within the definition of munchkining provided. Assuming for the sake of argument that it's good advice, I think people refrain from following it more out of akrasia and to avoid the bad feeling of losing than because they don't see it; think it's impossible, unsporting, etc.