Personally I see no reason why gliders (and older aircraft with little/nothing in the way of electrical systems) should, generally, need transponders. Especially when operating OCAS. However...
It's a whole different story when they start flying in cloud. If I'm flying along in IMC in receipt of a radar service I do NOT want something invisible to SSR anywhere near me. At the very least a charlie-capable transponder squawking a discrete "I'm a glider in cloud" code that can be switched on before they enter IMC would be a good idea, surely?