If I remember correctly Rise of Nations had an excellent feature that actually penalized focus fire. You could still focus fire of course, but your attack was cut in half (or something similar). As I see it penalizing focus fire has the following benefits:
1) It is more realistic. 10 Soldiers do not focus fire on 1 soldier in real life.
2. It improves unit balance. Cavalry would actually be able to kill skirm without the lame retreat – focus fire – retreat chimpanzee “tactic” screwing up unit balance.
3) Games would actually be won by the player who thinks the best instead of the player who clicks the best.
I expect to get flamed for this since ability to click fast on the mouse is the only thing separating most of the current so called experts from intermediate players.