I guess this may be well known. But looters are actually fantastic for leveling up troops I've found. You need to send your troops though. Looters work well on two fronts. First, and most important, is that on an auto-resolved battle, no one dies against looters, secondly, it seems to me, that troops level up more rapidly in auto-resolved battles than real ones. Maybe due to the player taking kills. Very very handy for those "train up some peasants" missions.

