You can grind renown to a large degree and not get annoyed by offers to become a vassal which hurt relations when you decline them. You get paid based on a portion of your party wages and it scales up as your party grows and becomes more powerful so it is often more profitable than being a new lord with just a village fief.
There are a number of things that are good about being a mercenary. The few times I tried it, it's just sit and watch, and eventually get bored while you're at it.
Originally posted by jml:I never really got what's interesting about that mercenary thingy.