Gliding competitions in the UK often involve up to 100 gliders; if all these used transponders it would mean the radar display being unnecessarily cluttered.
As far as primary radar goes, if it's 'raw' primary with MTI, you would lose the blip every time the glider circles in a thermal as its turn takes it tangential to the radar (tangential fade = zero apparent groundspeed); if using processed primary with an adaptive MTI system, it would depend on the threshold speed of the MTI, this being the groundspeed below which all returns are filtered out, usually 50kts but tends to vary according to user requirements.