They are called winglets. They reduce drag caused by high pressure wing spilling over the top of the wing into the low pressure area below. You've probably seen pictures of airliners with contrails forming at the wingtips. The winglets smooth them out. They reduce drag/fuel usage by something like 7%.

No reason they can't be used in RC with the usual concerns (weight, complexity, etc.) taken into acount.

Here is a link that goes into more detail ...

http://airtransportbiz.free.fr/Techn...hewinglet.html