This feature would be purely for appearance and is obviously not neccessary.
I think it would be neat if troops could be toggled to a "mingling" setting where they simply wander around the city, perhaps looking busy, and essentially just make the city look more alive rather than having them all just stand in one place doing nothing.
However, when the time comes that the troops need to be gathered, no one wants to go around looking through the city for all the scattered troops. So, I suggest that there be a player designated rally area so that when a player selects the "Rally Troops" option, or whatever it would be called, all the troops would gather at the rally area for easy selection.
Ohnotord Femeas, Lord of Dsrea Anud