I too fantasize about joining parties. I think with this extra option, one might find a better place to fit in during the beginning, and in the late game, it can prove to be a much different challenge than commanding 45 knights.
I think you should be able to join a troop, and get pay, based on level, or number of kills. Your pay should be part of some sort of contract, that if you breach it, and leave the troop early, you become a fugitive, or labeled a deserter.
This also might be an interesting way to move up the ranks to command your own troops, rather than command an army from the get go. Build up through promotions.
tell me what yall think