I agree with handofcod with this. Whilst I've been in some situations where having a knife would have made me feel better, I think thats all it would have done. Especially the part about flashing it out of fear, the other persons involved would more than likely see it as a challenge, and maybe not know whether you are scared/looking for a fight.
However, at least we can carry it whilst hiking/camping, or if we have a legal reason/job like a paramedic, or if you've been fishing, but thats where it all gets a bit shady.
not trying to make my post too political, but I wonder what would happen if you picked up a package from the post office, didn't open it, put it in your bag with, and knowing that it had a knife inside, if they would charge you with concealing a weapon? I was in that situation the other day, no police, but still wonder who would be in the right. of course I geuss you could say you didn't know that the package had a knife inside.