I use merc armies if I want to cripple or destroy a faction, but do not want/need their settlements. I send in one family member that has some potential, take the enemy settlements, exterminate their populations and give them as a gift to an ally (or just let them go rebel)

Al Kalhlifia, I like your stratagy of taking out Rome just so the other Romans kill each other. Taking out the Senate army with all mercs is an impressive feat.