Problem is they're pumped full of adrenaline and stoke up on goofballs, so more often than not the aggressor relieves he victim of the weapon and uses it on them.
Leaving aside the legal problems, the physical reality is that unless you're skilled in its use its not liable to give much if any, advantage.
Interesting factoid. In the 1980s the FBI decided to update their defensive training for their agents. They started from scratch, and analysed tens of thousands of violence reports to see what actually worked in the real world. While there were occasional examples where a midget with Bruce Lee skills or a granny with a shotgun might win the confrontation, the only factor that gave a statistical above average advantage was muscle mass. Good old fashioned brute horsepower.