Most trainers use the rubber band method. Actually, it's probably the best way to hold the wings down.

The purpose is to have a quick break-away method of the wing in case of a bad landing. The rubber bands will allow the wing to break away with minimum damage.

Bolt-down wings may look better, but they may take more of the fuselage with them in a bad landing, thus causing more damage than would a rubber-band held-down wing arrangement.

CGr.