Using a short range radar at an airfield (ie out to about 40nm), they show up pretty well as primary contacts but not all the time. The return will tend to twinkle as the glider manoeuvres due to large changes in the glider's aspect to the radar head and its ground speed.
Transponders in gliders would be of use when they are operating in small numbers but only if they had Mode C. They would be of no value during mass competition launches; the overlapping labels would just cause ridiculous clutter.