Don Johnstone wrote:
Unless 100% of gliders have
it installed it cannot be effective

If 99% of gliders had it installed, and there were 100 other gliders
flying in the area, the risk of not being aware of one would be
decreased by 99%. If 50%, risk decreased by 50%.

Not as effective as if 100% had FLARM installed, but surely not
completely ineffective?

I'm not (yet at any rate) planning to install FLARM for flying in the
UK, but would if I flew in the Alps. I can use a 50%+ improvement in my
chances of spotting other aircraft.