Absolutely on fixed blades, nope on folders. I have had many, many of my customers make sure I was putting one on. Mostly they use the term "thong" or leather string. Have never heard fob but occasionally lanyard, mostly thong though. I have never had one tell me not to put one on their knife. My sheaths fit very tight they are made for active, horseback, cowboying use. The thong is advantageous in helping to get the knife out of the sheath. Kinda catch/brace the pinky against it to help draw.
Half of the Christmas orders last year done and working on the sheaths. They all have thongs:
Helps getting em out of the sheath, particularly in a hurry:
For us a good sharp knife, handily carried is not only useful, its a safety item. The difference between literally life and death or very serious injury. I've personally cut three folks out of bad wrecks involving horses ropes and cows and two almosts, myself included in one of the almosts. I've taken many a phone call the day after a wreck saying I need one of your small fixed blades. I needed a knife NOW and couldn't get to my folder in my pocket under my chaps. Never once has anybody said don't want the thong. Always hear make sure ya put one of those thongs on there.
Granted this cowboying deal is a small subculture of the world, but this has been my experience.