This is from another discussion:
The professional soldiers and officers do serve in regular units.
The idea is that when you have them, they free regular soldiers and officers from the army to join the work force allowing you to have more corporations.
In this way, building the amy does not take away people from the economy. Before, a large army would rob the economy of many workers and medium level managers and damage the economy.
The down side is indeed, that they will die in wars.
The exact numbers are available under the Army menu option --> general data.
I agree that making it optional may be helpful.
I will check if this can be done.