Mounted cameras are very useful for post-flight debriefs for low-hour (<350) pilots.Not only for crashes, but for anything that didn't go well - poor landings, mistakes in ATC comms etc. Quite often the learnt lessons are shared, so it helps others not to repeat the same mistakes. Look up for example Flight Chops on Youtube, he also uses 3-5 cameras, but of course ,not all aimed at him
instead they capture the cockpit, instrument panel, hands & legs, sideways, from tail to nose etc. So I dare to say that fixed cameras add to the overall safety of private flying.
Taking photos with a handheld camera is a greater risk, but passangers appreciate it, and they fly for fun too, there has to be a picture from their flight. Yes, there is a increased risk with them (dropping on the floor, jamming control, dividing attention), but c'mon, navigating with a paper map, folding it left right center is just as much distraction from wathing oncoming traffic or the airspeed. So if you can't prioritize flying and navigating, something else will catch you anyway.
