In streamer combat, all wings have vertical stabs for one BIG reason. You might be stable running around at first. But snag 30' of crepe paper on a wing tip, and see how stable you are NOW. We also don't have rudders, so you need something to overcome the yaw from all the extra drag coming from who knows where.

For foamie combat, and streamer combat to a lesser extent, you need that vertical to get out of a flat spin induced by contact with another airplane. In the Open B class, I've hit wing-to-wing head on at about 150mph closing speed. This results in the coolest, fastest, and flattest spins you've ever seen. Getting out of them with out a rudder takes a lot of vertical area.

And finally, tons of vertical area allows you to fly closer to the stall with out snapping. When turning radius is the name of the game, big verticals allow you to turn tighter. I believe this works because the large vertical area works as a yaw damper, and gets the wings to both stall together. Coupled with an airfoil with a soft stall, you get a plane that can fly right at the critical angle of attack, or slightly beyond it, and not snap or fall out of the sky.

A great exampe is the Falcon, designed by Lee Liddle. It's a flying with, with a moderate taper and no sweep at all. It has winglets and a small vertical tail on a boom. I saw the vertical tail get cut clean off, and the plane stay in the fight. Right up to the point where it scored a cut way out on the wing, and the yaw that resulted was rather comical.