The enemy conquers one of your valuable border cities, you end up being able to defeat them and begin to retake control of the city. Your troops should be pissed off about getting back control of these walls to guard the lands from the enemy? They should be pissed off about leaving the city in a condition that makes it able to recruit more forces to help them defend their homeland?
Your suggestion may make some sense if you purely think of conquest, but makes none when you consider liberating regions that were lost. After all, not all men are blood-thirsty savages, either.