Have you seen a pizza-box flyer (PBX)? It's a flat square with a vertical fin, elevons, and a motor. Go ahead, search on the term and take a look. Anyway, the rudder is a more powerfull directional control than the ailerons - but the ailerons are there because pilots are more used to steering with them. A PBX could easily be flown without dihedral or ailerons. It's dihedral EFFECT that's important, not just the actual angle.