It doesn't seem reasonable to me to evaluate if a law is necessary by checking how enforceable it is. Even if a crime is hard to fight, I think that's a problem

It doesn't seem reasonable to me to evaluate if a law is necessary by checking how enforceable it is. Even if a crime is hard to fight, I think that's a problem but not an excuse. And if the problem is the police singling out some people like you said, then in my opinion there are other, more straight forward ways to tackle it. There are too many cases where a crime should be prosecuted even if it's difficult to do so. (As an example, I consider rape a difficult crime to deal with, but I wouldn't understand not prosecuting it because of that)