I never said I disband them afterwards. :) Especially when fighting in barbarian lands, I only disband mercs (who are relatively cheap to maintain and fairly effective, anyway) when their numbers get too low (say, if their numbers fall into the single digits) or when I really need the population to prop up an emptied-out town. I hate it when I enter a barbarian stronghold, only to discover that they have bled the town dry in the process of fighting me. There's really not much you can do with a town with, say 500 people left, except to sit on it for a inordinately long time.