Well, the anti's say we don't need "weapons of war" (I know the AR-15 is the civilian version, but bear with me), but the Founding Fathers made it clear in their papers that in order to deter defend against invasion or tyranny, the people need to be armed with weapons of war.
Why do we need these "high powered assault weapons that enable lunatics to mow down innocent kids?" Because the features that make a weapon attractive for mass murderers also make them attractive for self defense...a use that the Supreme Court has upheld. Ease-of-use, lightweight, low-recoil, high capacity makes for a good option for your 16-year-old daughter when she's home by herself and four guys break in through the back door. I'll also point out that (if it were possible to make criminals follow the law) reducing the legally-available features is a bigger issue for your "prepared enough" citizen than for your prepared attacker. The citizen limited to 10 rounds will have 10 rounds in the gun instead of 15. The attacker will have so many loaded magazines in his backpack that it's a non-issue. So when someone says a law will stop crime or reduce the number killed in a spree, but it will not affect the citizen defending itself, call bravo sierra on the spot.
Why do we need the same weapons that the police use? Because we fight the same criminals, except we are the ones caught by surprise and we are the ones without backup. Most of the time, the police respond after the crime has been commited, and are more focused on enforcing the law that was broken instead of preventing it from occuring. The supreme court has held that the police doesn't have a specific duty to protect any individual, only I have the responsibility for myself.
Why do we need weapons with no "legitimate hunting or sporting purpose"? Well, the idea that hunting/sporting purposes are the only legitimate reason to own a weapon ignores what the Founding Fathers meant with the 2A, and the semi-neutered version that the Supreme Court gives us today: protection. However, these weapons are used every day in hunting, due to their innate accuracy and the ergonomics of the platform. These are used in target and action shooting competitions, giving them a legitimate sporting use. To say they have no use in these fields, and therefore have no use, shows such ignorance on every level of the argument.