Uh, I think building the units represents training them (since I don't think they are actually putting soldiers together in the factories). And as I said, anyone who thinks any training time should equate with combat time is way off the mark and I think it might even be construed as an insult to combat veterans. And if training time equated with combat time, 2nd Lieutenants wouldn't be the brunt of so many jokes.....
The experience promotions should be seen as a perks for units putting themselves in harm's way and living to tell the tale (and fight better next time based on the experience they learned in battle).
Plus, it's cool when in modern times you have a buffed up unit that has been with you through all the Eras (and you know this because he is buffed up). If you can just grow your own buffed units, it takes away the specialness....