<-- Aerospace engineer... I would just think that the wingtip vortices would create some kind of interference there with the open tip. Hmm... I'll to look into that some more.
I work with jet engines, not wings... 
That is a simple light weight, electric R/C plane. Wing tips aren't critically necessary. It could get blown around the slightest bit in a cross wind, but not too bad.
I had one once upon a time that the design was basically a simple spar construction wing mount to a piece of 2" square stock pvc. Simple design, but that plane was very fast, responsive, and durable as all getout. "Real" engineering really isn't too important.

In fact, the crap R/C planes and heli's can do would probably make alot of engineers heads explode due to all the physics laws they'd be breaking if they reacted like their big brothers.
