I say again - if you are serious about your flying, and about safety, buy a PCAS!
If money is no issue, I agree. But between PCAS, Flarm, GPS, PLB and the spare Icomm you can easily spend half your PPL costs on electronic safety-enhancing gadgets alone. Alternatively, you can spend that money on advanced training such as aeros or an IMC.
I don't have statistics to hand, but I don't think mid-air collisions are very high on the scale of hazards in light GA. Loss of control (usually in IMC, plus departure stalls and stall/spin accidents on final), or controlled flight into terrain (usually in IMC as well) seem to be killing more pilots than midairs.
But gliders would have difficulty in powering a tx for a 4 hour flight, microlights, similar.
Actually, virtually all gliders in the Netherlands are now equipped with Mode-S transponders. Where I regularly fly they're not operated properly, in the sense that they're set to "Alt" as soon as the first pilot connects up the battery and performs the A-check, and are only switched off when the battery is disconnected for recharging at the end of the day. So they're running all day on the same battery that was previously only used to power the radio - which weighs approximately a kilo. So the electrical demands of a transponder are not that hard to meet.
What is a bigger problem, is installing the transponder antenna in a modern carbon-fiber glider. Since it has to be outside the hull, and causes drag.