Absolutely, it should remain legal.
There are a few weapons that may be even better for defending the security of one's State but the AR-15 is definitely in the top 10.
By defending the security of one's State are you referring to fighting our military should our government turn on us?
That's only one of the several possibilities where weapons can be Constitutionally used to defend a State and it's security but the short answer is yes.
Yeah, I hate to break it to you but if your state goes to war against the US military, there is no number of guns that is going to help your cause... We've moved way beyond the days of muskets and swords.
If you want to look at real world problems you have to look at the violence on our streets and how we protect ourselves and how our officers enforce the law. Sport and hunting are secondary bonuses that come with gun ownership.
The thought of any State ever having to defend itself from our own military (again) may seem far fetched and one sided to most. However, it's not simple and it's not as cut and dry as one might think.
Look at the problems our military is having (and has had) with the armed citizen/ militants in Iraq, Afghanistan, Syria, etc.
Then to, you have to imagine our armed forced being ordered to fire on their own citizens. When I was in the Marines, that would have been considered to be an unlawful order. No doubt a significant number of soldiers would sooner defect and take their weapons with them to join in the opposition to any U.S. government that even tries to attack its own people with the military.
The right to defend ones self, our homes and our businesses is a given. We don't need a Constitutional Amendment to make sure the Government knows we have that right.
The 2nd Amendment was not written for us to defend ourselves against one another. It was written to secure our right to defend ourselves against the government itself.