Regardless of the time period and circumstances, it is always bad to have an enemy held fortress behind your lines and risking your supply and communication lines. As a result, it is very important to either leave a token force to besiege a fort or take it by force.

The very idea this is why many countries developed many fortresses along old regal borders during the 17th and 18th centuries with the Vauban system of fortresses doing the same thing. More so, San Antonio is blessed with an abnormal amount of missions, designed to be safe places against attacks from native Americans in the area. So moving around a fort isn't useful and can in fact be very dangerous.