I carry traditional slip joints daily, and am also amused by the "need" for a lock.
Don't get me wrong, I have locking folders that I use. I carry my locking folders as a "I might need a knife for non cutting tasks" (specifically I carry locking folders as back up to a gun).
I have never cut my self with a non locking folder (as an adult) where the lack of a lock was the reason for the cut.
I cut my self all the time on them, don't get me wrong, I play with traditional folders with strong locks and sharp edges enough, that I nick my self occasionally.
All my serious cuts have come either from a locking knife, or a fixed blade.