And what you aren't getting from my words is the question of "What significant improvement would this bring that addresses a significant flaw in the current system?" You bring good points in terms of "realism", but I am not sure if you are maintaining awareness of how the playerbase as a whole would take it. Sure, people can adapt in time, but how much of a difference would it really make? What exactly is so lacking in the current way that training describes the unit's experience, cohesion their ability to work together, and morale their mental state?
Are we to suggest next that we should add something else to account for random in-fighting in units, hence a "Dissent" stat? Maybe we should add a "Psychological Stability" stat to see which units can resist losing morale? Or a "Liver of Steel" stat to see how much entertainment you can give them. Oh, surely there are Olympic sprinters in some infantry, or Kentucky Derby racehorses in some cavalry, let's give a "Speed" stat. Shall we have "Navigation" to determine how well they find good/bad routes on travels without scouts? These are extreme examples, but when it comes down to it, I'm sure someone out there can justify those aforementioned passionately.
In other words, how would you justify the potential benefits versus the added complicating factors in an already unpredictable combat system?