For home defense, I don't think its the best choice either. But if you live in an area hostile to guns, I think there's a case to be made for using an 870 over a scary looking rifle. More generally, shotguns are just awesome guns. You can hunt squirrels with a 12 gauge and turn around and use the same shotgun for bear defense with slugs. It doesn't get more versatile than that. Plus the shotgun games and the fact they're just plain fun to shoot. And even during buying panics you can usually find shotgun ammo at Wal-Mart. You can buy a used Mossberg 500 for peanuts and its a fine gun that will last forever.
I think its a decent HD option for those who are resource constrained, plan to only have one gun, or live in a socialist utoptia. But most P-Fers would probably be better served with an AR.