There are really three reasons I
1. The AR-15 is America's rifle. It fought and continues to fight on fronts all over the world in the hands of Americans. It is a symbol, and has served this country well.
2. This is really an argument for carbines. Having a short-to-mid range, rapid fire, carbine with decent penetration is the yin to a shotgun's yang. Even the largest, most intimidating, room-broom still has disadvantages. Is there a perfect, well-rounded weapon that has a balance of both worlds? Probably. But I'd rather have selection of two weapons that are optimized for different situations than pick one that will have any shortcomings in the situation I need to employ it in. Like settling on a wagon when you really need a small, fuel efficient car, and a large truck.
3. The SHTF situation. If America becomes occupied or threatened by forces foreign or domestic, there's about a 50/50 chance they will be carrying weapons that use 5.56 (.223) ammunition. Which will be extremely important in mounting any kind of resistance. What's the other 50% chance? They will be using 7.62 ammunition. And yes, I am looking into getting an AK type.
Some have said they do not understand my third reason. I was tactfully suggesting that we may, one day, have to turn our weapons on our own government (in the form of our military), and they will undoubtedly be using AR-type weapons which will fire the same ammunition as my AR. The decision was a question of ammunition availability in the case that ammunition is taken away from citizens.