I'm all for NOT exploiting the game mechanics, and the way the replacements work now is wide open to exploits.
Still, it has one big advantage :
You build up one unit, you watch them slowly grow into an elite to be feared, you become attached to them... Then, one battle, you do something stupid. Like letting your uber 9exp vardariotay chase some routers, then forgetting the skirmish off only to find them later almost wiped out by some lowly militias gangin up on them. When that happens I send them off (secretly) to the nearest retraining center. And voila!, my gold unit is back ! And then I pretend the stupid mistake never happened

Eh, just keep hitting M after battles and only retrain the leftovers...