I have a SPAD (DASPLASTIC) that had open wing tips. I covered the openings with tape because it just didn't seem right. Other guys at the field fly these planes with open wing tips just fine. Does it really make any difference?

No. It does keep dirt from getting into the flutes and rattling around.

Hmmmm... in knife-edge flight, it seems you're losing some surface area

Arn't they so ugly that conventional aerodynamics don't apply??

I've the reason these fly so well is the ground repulses them!