It has nothing to do with "need." It has to do with "why would I allow a govt to tell me what I can and cannot buy?" In the same respect that Obamacare compels us to buy insurance and it's ok, why is it ok to tell me what I can't buy?
Banning 30 rd mags but not 20 rd mags is saying "We're ok with you wiping out 20 people with well placed shots but we draw the line at 30." Banning 20 rd mags but not 10 rd mags is the same silliness.
This kind of debate is just posturing on the side of the Liberals. It's entirely immaterial what is available to the law abiding citizen because the we don't break the law. The criminal, by definition doesn't obey the law so the laws don't apply to him. But because we follow rules and the govt can't take out it's frustration of being impotent to control crime, they make themselves feel better by lashing out at the law abiding citizen.
Look at Australia, Mexico and all other socialist countries. The law abiding citizen is relegated to a .380 while the cartels and gangs, etc have more firepower than their military. How does banning a 30rd mag prevent a criminal from going on a rampage? It doesn't.