From my minimal experience I feel that on teams that don't have high stats (ogres, goblins, etc) you'd want the extra runner as your reaction plays aren't really giving you an advantage. Whereas teams with high stats (Pirates, Amazons, etc) would benefit more from having more kill zones to take advantage of those higher base stats in contests.
This is exactly my line of thinking. Teams that are more defensive in nature will not be initiating contests all that much.
Goblins you always want more runners because of Scatter. I think the rest of it really comes down to what your upgrades/ringers are, and what your opponent has access too.
But subbing a bruiser for a Ringer is fine strategy if you have a plan to utilize them.