@Intrepid,
I enjoy shooting and wanted to add another gun to my collection. I had already owned a handgun and a shotgun, and I thought I'd add a rifle.
I chose this particular rifle (AR15) because of the magazine capacity and when I'm shooting at the range I don't have to reload it near as often. Also it is a very accurate rifle, which is important when target shooting. Also, the resale value of this particular gun has historically been very high, and thanks to Obama's election, the gun is actually worth several hundred dollars more than I paid for it. Not a bad investment.
Another reason I chose this rifle is because the assault weapons ban had expired, and I wanted to purchase a weapon that was previously banned to send a message to our politicians that there are many people in the USA that would NOT support another ban. Sort of voting with my dollars. I believe that the more of these weapons that people own, the less liklihood that the government will pass another ban. And if they do pass another ban, then it's likly that my weapon that I bought before the ban would jump up in value (this happened during the last AWB as well).
I bought this during Bush's presidency, and part of me thought, at the time with Bush passing the Patriot act and a lot of our rights being trampled on, that it may be necessary to fight to protect our rights. Then Katrina happened and after seeing how society can breakdown during a crisis, I felt very relieved by my purchase.