Sounds like a good idea at first but there are too many issues with it:
1. How will the AI cope?
2. How are the barbarians affected?
3. Are the battles big enough to require such macro-organisation between the commanders and his units?
4. How annoying will this be for the player?
5. How would units meant to act cohesively (i.e. the phanlanx) act if some parts of the line refuses to move?