This permits us to defend ourselves when the police can't or won't, and it permits a residue of firepower in the hands of the people with which to stop any tyrant who might try to infringe upon our natural rights, and it will give second thoughts to anyone thinking about tyranny.
The intent of the second amendment appeared to be to ensure that the people could resist any attempt by the then-government to push things too far and become tyrannical. People are still debating that this is a valid reason to own an assault rifle.
Does anyone really think that a government, with even partial control over the US Army, USMC and National Guard, would have any issues stopping a handful of wannabe-militiamen with AR15s? It's so far diluted as to be meaningless, that interpretation.
The country is ablaze with passionate debate about guns, and the government is determined to do something about it. Debate over public policy is good for freedom. But the liberal progressives want to use the debate to justify the coercive power of the government to infringe upon the rights of law-abiding folks because of what some crazies among us have done. We must not permit this to happen.
The whole purpose of the Constitution is to insulate personal freedom from the lust for power of those in government and from the passions of the people who sent them there.
Must say I've enjoyed fellow Brit Piers Morgan's recent work on the subject, so I'd present his main case to you for your opinion. You are not going to take down the government, no matter how many AR15s and their ilk you have in your basement. If you hunt, you are equally effective with a single-shot bolt-action rifle, or a shotgun. If you want to defend your home, you'll find a .45 or 9mm handgun much more effective to use and manoeuvre with in a tight space like your own stairwell.
Shooting for sport, hunting and self-defence is better served with different weapons, and there's no hope of you being able to take on the National Guard as a militia. I live in America and have watched the news reports on the shooting in Albuquerque and Sandy Hook. What possible reason can there be for allowing normal citizens to purchase military-grade assault rifles??