My problem in Argentina is that we don't have properly defined knife laws. Our weapon laws focus only on firearms and knives fall in a very unclear category of other weapons (weapons being anything that can increase your offensive potential as far as the law is concerned). Carrying any weapon is illegal, so most people (and this includes police officers) think knives shouldn't be carried in the street.
You can argue a defense, but you get your chance to do that when you go to court and your knife is long gone by then. My father is a lawyer and he works as a public deffender (the guy you get when you can't afford or don't to hire a lawyer of your own). He gets a gazillion cases of idiots arrested with crappy knives and he usually gets them off, but he told me that those guys are the ones that create a problem for responsible knife owners. He looked at over a thousand confiscated blades and he couldn't find a single one from a good brand or maker (and he knows knives).
Our law is so ridiculous that people like my dad and I who have a permit to carry a gun can't carry a knife!
The current law is supossed to be open and allow you to carry anything as long as you don't have any violent intent (and "intent" is the key word here) or a plausible explanation for carrying it. Who decides wether or not you look like a guy about to commit murder or if you really need to carry that knife for your daily tasks? The arresting officer, of course. The problem is that our police force is so corrupted that they are actually the biggest organized crime group we have. Sometimes looking respectable will only make you a target for a bribe request. Of course, there are some brave and honest cops.
Anyway, this is a problem for me, not because I fear being arrested (I don't engage in any kind of activity that would lead to a search and don't flip my balis around at the mall) but because I don't have the actual legal grounds to tell the sheeple to shut their pie-holes because I'm within my legal right to carry my knife.
Something similar happens with my proffessors. If I get a sheeplish teacher, I have to suck it up because he is the one grading my papers and deciding if I get my degree or not, and however unfair it might be if he is against knife and gun owners I sometimes have to hide my interests (there are some good teachers that are opossed to guns but accept an open debate).
Sometimes things happen that show I'm right about all the gear I carry and people recognize it would have been nice to have me around. For example, a few months ago my mother was in a court building and the elevator borke down, it came down two stories and the lights went out. The elevator was aboslutely packed with people. It took the firefighters quite a while to open the elevator from the outside because it was stuck between two floors. When they did manage to do it, they check out the inside and the chief said that a flashlight and a screwdriver would have been enough to get the doors opened from the inside. Can you believe that out of the fifteen people on that elevator not one of them had a flashlight, screwdriver (or knife to use as one in such an emergency).
Another example, last week my brother went to a park in the outskirts of the city. I convinced him to take a small PSK (mainly a first aid kit and some other gadgets), since they were going to play some soccer matches and horseride he agreed (somebody always sprains his ankle or stuff like that). A girl who was with two guys (none of them were friends of my brother) were running around and the girl fell, with such bad luck that the beer bottle she held broke and her forearm landed on top of it. Her arm was really messed up. They called an ambulance that took half an hour to get there. If my brother hadn't had his FAK and knowledge (he is a med student) that girl would have been in lots of trouble.
What does piss me off about some knife-haters is that it's actually in their best interest that I carry a knife. I once had to cut a guy out of his car (seatbelt was easy but I had to cut his boot too), how would I have been able to help him without a good knife? Chewing through his boot?
Sorry for the rant, but these kind of topics always get me fired up.