I don't see any issue with it added a bit of text to the unit's description information like - "Rookie" "Veteran" etc based on the total damage it's dealt/recieved, and/or number of kills. As long as it's just for the purpose of seeing how many kills a single unit has / how long they're been on the battlefield. It would actually be kinda cool, and give a little bit of uniqueness to each unit. But I'm against it having any stat altering effects. Purely cosmetic, basically a more advanced kill counter.
X :cool:

