The problem of promoted units mainly arises because a) many players don't train armies that give the entire spectrum of society (through financial problems or because they like to spam a certain unit) and b) most campaigns don't take a season or maybe two but several years due to the 4tpy which in turn makes it strange to run around with a unit of ... jugunthiz for 5 years as by that time they'd probably all be dugunthiz. With more tpy or realtime this "problem" would be somewhat removed as a campaign can end at harvest time.