How is it not a practical home defense weapon?
Lots of reasons. Your fantasy home defense scenario will happen on your terms with a gaggle of bad guys dashing down your hallway into your kill zone while you spray and pray. That's not a realistic scenario.
If you're honestly worried about self defense (and not some zombie invasion thing) then take a couple good pistol classes and get yourself a CCW. A good AR loaded with the current best self defense ammo on the market won't mean anything if it isn't in your hands when you need it. An AR carbine is a great ranch rifle and you can certainly defend yourself with one (no matter if it is loaded with 55fmj, 60vmax, 75bthp, 52bthp etc) but you're going to look a little bit strange carrying it with you wherever you go.
edit: Teach the wife and kids how to shoot and make sure they know where you keep your home defense weapon as well (as long as they are old enough and responsible enough). Lots of home invasions happen when the parents/adults aren't home - just recently here in Sacramento some 13 year old girl and her baby brother were damned near stabbed to death by an intruder who turned out to be a 14 year old boy.