Quote Originally Posted by Thaatu
It's a moral question whether one chooses to destroy enemy barracks. Bear in mind establishing a military recruitment system isn't done in an instant.
You should always destroy any barracks that you don't have the ability to use. It's unrealistic if the original owner retakes the place and can immediately recruit elites. Re-establishing the system might be easier than establishing it in the first place, but you've lost much of what the system consisted of as the place was taken by invaders.