Yes AR's and other semi-auto assault like weapons are used in less crimes, but they are more often used in mass killings. You said it yourself, against a crowd of people the AR is the gun to use. Also, if the government were to attack us ( the citizens) the AR's are pea-shooters compared to what hey have. We are a highly developed country, our country isn't going to attack us. Protection from the government is a lame excuse to keep these guns legal.
Not the best example but an example where the people shot were only wounded by a shotgun. You have to be pretty close and hit someone in the vitals to have a high chance of killing them with a shotgun. So close that it would be much easier for the bystanders to engage the shooter with force. Thankfully this kid was talked out of continue to open fire. This happened today by the way.