If banning ARs prevents even only one single school shooting, it would be well worth it.
Most people on the left, at least the ones I talk to, realize banning assault rifles would not stop mass killings. However, isn't it easier to kill a lot of people with a big gun than with a smaller gun ? Out of the 4 deadliest mass shootings, 3 where done using an Assault Rifle (Sandy Hook, Vegas, Pulse Night Club). Isn't it easier for police forces to arrest someone using a pistol ? (Less suppressive fire, less rounds, authorities are more heavily armed). I've personally shot both, much harder to be accurate with a handgun.
I'm really 50/50 on this issue. On one hand I believe the action of a few should not outweigh the liberties of the many. On the other hand, fully automatic machine guns have no practical purpose other than leisure (in my opinion). I would gladly sacrifice one of my hobbies if doing so could limit the number of dead.
Also, the problem is that the term "AR" is used in a very loose way. I have a problem with fully automatic weapons. Now, I know what you're going to say : "well you can make your own bump stock, so banning automatic weapons would be pointless!" to which I will answer this : You can make a bomb with a pressure cooker, some salad oil, ground up rubber tires and toilet paper. Does this mean we should sell bombs ready to go since "you can make your own if you want to anyway" ?