Paradox should add "switch" for rulers to prevent them from leading armies. Council members already have them, so why not king?
This is a good idea.
Excellent idea.This could even make a nice event line; your liege summons you for military service, do lead your troops, send your troops with the Marshal, send them with a random courtier or even refuse to send them. All options have impacts to your relationships with your liege and vassals... If you agree to go you (or whoever you send) can not be recalled for a certain period of time (say a month)...