Rezca
The Greatest Jaggi
May be I don't understand...But I always doing so. It is a simple hack - right after you get siege war build 10 catapults, go to the nearest barbcity and get as much free experience as you can. Do not kill barbarians if you can, it is source of experience! They can get 20 experience each or even more sometimes => 4-5 promotions. Also it is tons of scores for field generals. Also it many scores for great generals => more field generals. It is like the training before the real wars, it is necessary to not suffer losses in wars with civs.
It's silly to me because the AI is actually trying to kill these units, not 'practice'. I've even witnessed a lone catapult spending more than twenty turns trying to kill a barbarian Lion that got trapped in their borders!
I understand the bit about it being useful for Commander units, but I've since stopped using those since it felt like cheating to me. The AI almost never uses them [properly], and enabling that option means they rarely seem to settle them, merge them with troops (Led by Warlord), or use for Acadamies. These Commanders they end up using as scouts, or have like three unpromoted Commanders huddling in a city, while I have a Commander-led army that has virtually every promotion available to the Commander and is walking from city to city razing everything and all the bonuses rendering the stack untouchable.
Field Commanders are a great idea in theory, but right now the AI just seems to have no idea how to use them, always converts their generals into Commanders, then lets them go to waste.