I don't know. Imho that's the same logic you hear from people where they say something like "I'm a safe driver, I've never had a accident so why do I need a seat belt?" There is still risk involved.
But on the flip side I would argue that a person with a slip joint usualy would be much more mindful and cautious when using a knife then a person with a lock which can be just as safe if not more safe.
The only problem is there is less and less people who know safe knife usage. Heck in my area I've heard even the Scouts are talking about removing knives from the program.
True...and that definitely makes sense, but with the driving analogy you have to take into consideration that other people may not be safe drivers. So there's a gamble trusting other people on the road. I think that slip joints are good in controlled circumstances. If I'm out on the trails with my boys and we need to make a repair on a truck that requires a knife, I'm probably gonna use something with a lock. But that circumstance is so few and far between I wouldn't only carry a locking knife because it. I have a couple in my bag for that