It's more fun to kick their arses in combat than sit around waiting for them to die of boredom. Unless you're carrying lots of tasty morale-boosting treat for your own army, you'll probably start to lose troops.
Mmm, pass me more butter, honey, sausage and dried fish stew, Jeremus*, we're going to be here some time, seeing as town supplies will last them 100 days.
*I figure Jeremus probably has to wash-up after doing the cooking as well, being the nerdy one. "Wah wah! Artimenner's always picking on me!"