I don't see how random promotions are more realistic at all, it would make about as much sense as only being able to produce a 'unit' instead of picking one, especially since the gunpowder-modern era mainly distinguishes units by promotions. There's no such thing as a 'mountain infantry' unit in-game, but put geurilla promotions on a generic infantry and you've got one. There may not be antitank guns, but put ambush on an artillery or machine gun and there you go.
I would find it unrealistic if somehow my units were trained in a barracks, directed by the Pentagon, and with officers from West Point, but that I couldn't offer the least direction in their training. Plus it's pretty clear that choosing promotions is supposed to be as much a part of building a military as picking what types of units to build.