As this may very well be applicable to a campaign I'll be running shortly, I thought I'd chime in.
While I think that the soldier working his way through the ranks to command troops has its place, I'd have to say that I'll probably lean more towards the noble/bard route, simply because there is a lot more to that post then simply leading troops. A good general/commander has to survive life at court and needs the related skills that the common soldier just won't have. He/She needs to be able to negotiate and compromise (diplomacy), and recognize adversaries and allies (knowledge heraldry, or bardic lore) to get what he/she needs to do the job. To be effective, I think you'd also need a horde of knowledge skills too, like logistics, tactics, seige tactics, geography etc, and in a D&D world, probably a little arcana too.
Now, this is probably a little too much realism for the average campaign (a fault I've been accused of more than once), but I'd still favour high skill point classes. And of course its all dependent on the social/polictical structure. If the kingdom is unfied against a common threat, for example, there'd be a lot less politicking and a lot more need for capable fighters to inspire and lead the troops directly. If it's a tightly controlled aristocracy, they're sure not going to want the common man in a position to have the loyalty of a large number of troops.
Of course, following this method you still end up with a fairly large number of inept commanders, which is a fairly common theme through history...