Yah i dont get it either...look at it like this...you roll in 50k soldiers lose 5k or so and you gain prestige and exp. for your hero....on the other hand you roll in mass ballista, take no losses, what you get is a flawless victory, but you don't get prestige. Looks to me like it takes care of itself.
Anyways, there are just way too many things that don't make much since in this game as of yet.