Spydutch,
first of all, thank you for the title of the thread. It's nice to be considered a "resource" in anything, in this big old styled family of ours

Now, back to your question with my two cents.
The short answer is, I never experienced any accidental opening of a friction folder while inside a pocket, nor heard of any, nor seen it happen. But, this question deserves a more elaborate answer.
If you take an Opinel (I use it as a common and known reference for friction folders), when the knife is closed it is very unlikely that anything can catch the nail nick or grab the blade and open it. And I would bet that, even before the locking ring was introduced, 99% of French farmers never even though of carrying their friction folder in a slipcase.
The right amount of friction in these knives is (to me) quite a factor. Too loose can be on the "risky" side, and too stiff makes no sense. I would consider Opinel's a "medium-to-loose" friction.
At the same time, some modern folders (usually lockbacks), which have no friction at all, can b worse depending on the mechanics of the back lock in the closed position.
It's pretty funny how perspective changes things.
I still consider backsprings (at least strong ones) more a danger (of the blade closing on my finger) than a safety (for the backspring keping the blade open or closed). That's why I sold my GEC #25: while it should have been considered an average pull by many of you, to me it was too hard and made me feel unsafe sometimes.
As a sidenote, I will email you about Sardinian friction folders, if you're interested in knowing more.
Fausto
