When you are nervous, it means you are unsure about your skill. The only way to be sure, is to know it forwards and backwards. Basic Training in the army is a couple months at least, the government doesn’t want you on the battle field until you have been trained – otherwise you are just a body with a riffle – and you are a danger to yourself and your comrades.
I agree with the notion that one should be well-practiced before performing. But I can't concur with the idea that nervousness is necessarily rooted in skill. It can also be caused simply by "stage fright", which can really only be overcome by performing.