Even in Vanilla, I never really understood the purpose of mercenaries. You can't field but a small amount of them, they're usually irreplaceable, and they're incredibly expensive when you do use them. Wielding an army of mercenaries means you're essentially cutting your army in half (or smaller!) for a less-than-double increase in power, so what's the point? What's the advantage to using mercs? Should I always use mercs, or is there a situation where they're just bad, or should I never use mercs?
I'm interested in it all.