I'm actually buying my first AR-15 this week (should ship today or tomorrow!), so I've been thinking a lot about "assault rifle" ownership - is it necessary?
Consider the following:
In every state where concealed carry (CCW) laws have been enacted - even in liberal, East-Coast states - crime statistics have fallen. I believe this shows that an armed civilian population is a better crime deterrant than an unarmed population. Assault weapons only shift this beneficial demographic further towards lower crime.
If the government outlaws the owning of specific guns or all guns, a criminal who's intent is already set to violate the law will not be deterred. He might have to pay twice as much money for a gun on the black market if the government really went all out in an attempt to remove guns from the US, but they will ALWAYS be there. I don't want my hands tied when I know the opponents' hands will not be tied.
Assault rifles are used in an exceedingly small number of violent crimes.
Assault rifles make excellent home-defense weapons (assuming .223 models, of course) - greater natural aiming and less over-penetration, meaning it's much safer to use in close-quarters defense, as inside a home.
Assault rifles are really one of the best ways that the population has of ensuring that they would be able to throw off a tyranic governtment. Pistols are no match against 21st century soldiers, should it ever come to that.
Assault rifles make great hunting rifles.
I add all that up my head and come to the conclusion that not only is it not irresponsible to have an AR-15 rifle, it's actually pretty beneficial, mature, and responsible.