In general glider seats are designed to accommodate a parachute. So you either wear one or use a (firm) cushion in its place. I expect the gent who had to vacate his glider after the mid-air with the Tutor near Didcot the other year was quite pleased that he wasn't leaning on a cushion. Ditto the two guys some years back whose glider was disintegrated by a lightning strike.
Re parachute training, I believe that it's a question of risk balance. e.g. I believe the RAF stopped doing single engined landing training in Mosquitos because more pilots were dying in training than in the real thing.
And I've seen three people die in freefall accidents (ok out of a lot of successful jumps and two of them were grandstanding low down)
Personally I have decided that the chances of my having to bale out of an uncontrollable glider are sufficiently low, combined with a reasonable chance of not meeting power lines etc on arrival on terra firma that I won't be practicing parachuting (plus the British Sport Parachute Association would say that I'm too old to start anyway!)
But each to his own, always. I'll keep strapping on one of those fancy backrests with a handle with the firm hope that I never see its contents.
