I can see from the point of view of PR it would work well and I agree with you and others who say it would make a small difference but when the next lunatic uses a bus or chemical mix or whatever what then?
If people put their energy into the cause, the multiple implementations won't happen and won't need to be legislated against.
To be honest given the state of affairs that the USA is in economically I'm surprised there haven't been more cases of these types of incidents. You're right, if somebody really wants to commit these acts they will, regardless of laws. They obviously don't care about laws if they're about to commit murder.
But if somebody snaps, or has a bad day at work, surely removing a source of destruction like a gun will prevent social damage in the long run. As Grinner said, you daren't get involved in road rage incase the other person has a gun.
It's not about banning anything that can do damage, buses/cars/baseball bats have a primary function which isn't killing, whereas guns are only made for one purpose; to kill.