I get why you might want to hit crouch a few times; if someone is aiming at your head and you crouch, all of a sudden, he's not aiming at your head anymore. I do it sometimes. But that's not what I'm talking about. I'm talking about people who spam it so fast their character doesn't actually crouch. I noticed while trying it out myself that my gun's accuracy and/or the hit detection was improved if I did it, but no idea why.