Leadsled, Mostly personal contacts and experience over 51 years and the stuff quoted earlier. Do your own research.
Having retrieved loose objects out of club gliders over the years which could have and did cause control interference I don't want to fly in them. The Brits were having quite a few spin in accidents when training about 10 years ago. One bloke posted on a newsgroup his disgust at the stupidity. The accident that caused that had a 15 year old student killed. It was the fifth over a few years. There was also an extensive discussion about this on the r.a.s. newsgroup where it was revealed that the Brits were initiating spins off a winch launch at under 1000 feet AGL. The American respondents were horrified "what do you call that" " insanity, officially sanctioned of course". I believe the BGA pulled their heads in on that one.
Again do your own research.
The better question is : why should people be forced to do something they don't agree with? The GFA sure isn't going to pay your widow compensation because some obese effwit insisted that you do spins and one of the halfwit instructors caused a fatal problem.
The common response when an experienced glider pilot is asked how the glider he flys, spins and recovers is "I don't know, why would I do that?" ALL have done spin training but few that I have met want to routinely do spins. It is a question of personal boundaries and risk exposure.
The ones who do have enthusiasm for spins are mostly on my personal list of "people who I won't be surprised if they die in a aircraft". I've managed to cross off quite a few.
I have no problem with owner maintenance for a person's private flying. It is a great idea as our Canadian friends have shown. I have a problem with owner maintenance where the maintainer puts the risk on somebody else and isn't in the aircraft at the time the other people are.