There's a big anti-stop-and-frisk march in NYC today. [View all]
So take this thought for what it's worth. While they talk about how the "stop and frisk" policy has nabbed X number of illegal firearms, aside from being both racist and an unreasonable search, the actual rate at which weapons (including things like pocket knives, illegal in NYC) are found is about 1% of all stops.
So a 1% success rate. Now in contrast, per Gallup, 12% of Americans report having carried a firearm at least some of the time for self-defense. The same percentage (non-overlapping) report having carried a knife for self defense. Not counting those who carry a knife as an everyday tool rather than for defense.
Crunch those numbers, and you'll realize that "stop and frisk" would have a vastly better odds of finding a weapon if they grabbed people completely at random off the street rather than hassling young brown men.