There are lots of reasons for owning an AR-15 type of firearm, but the most important is for self-defense in the aftermath of a Katrina like break down in civil society. The best defense against a mob of looters, is a semi-automatic rifle with a high capacity magazine.
While thankfully, these scenarios don't occur very often, they have happened often enough, especially around major cities, that a high capacity semi-auto rifle, such as an AR-15 or AK-47 makes perfect sense. I'm old enough to remember the many serious riots that occurred in major cities throughout the US during the 1960s. Mobs don't generally respond well to pleas for reason or mercy.
But even today, if one lives in or near a virtually lawless area, like the city of Detroit, a high capacity semi-automatic firearm also makes sense. Anywhere that gangs are a serious problem IMHO, necessitates the ownership of a firearm, capable of fending off mobs until law enforcement can arrive. In a city like Detroit, that could be the next day.