Ah, so I wasn't the only one who noticed the "transponder above FL180" mention that Jeb made, to which Dave replied with a not-so-convincing "yes"...
Agree with the points raised by Dave in the episode, transponder technology has gone long ways and relatively cheap and light transponders exist on the market today. This is not an issue with technology nor weight (sailplanes are not "that" light after all, LSAs are much lighter than sailplanes typically), it's just a matter of, guess what... money!
Why would a sailplane pilot spend money to install a system in his plane, when he does not get anything useful out of it?
In Europe, if you soar around the Alps, using FLARM is mandatory on the north side (Switzerland and Germany). FLARM is a proprietary system that provides an ADS-B-like system whereby each sailplane emits its position in broadcast, and other planes nearby can track and display on a small screen their whereabouts. More info on:http://www.flarm.com/
In the case of FLARM, people see the clear advantage of having this technology on board, and decide to buy it. In the case of the transponder, this advantage is not perceived and thus people will not want to spend money on it. Simple as that.
Interesting discussion, collision avoidance is always a hot topic, and accidents like the one that happened yesterday in the UK always remind us on how tragic things can turn when planes collide in the sky:http://news.bbc.co.uk/2/hi/uk_news/wales/7883338.stm