The price is right, but the gun is bizarre.
As for a second, weaker caliber in something like an AR15 or other rail system gun, there are a lot of multiple caliber guns through time and so it might be handy and a secondary caliber for dispatching wounded game that would not add a great amount of weight to your rifle (though the SBR thing applies as noted), but that would be a rather extreme suggested practical use.
I keep thinking I want one, but the controls are downright scary being located by the muzzle and there isn't any practial way to carry it currently.
"If you look through your scope and see your shoe, aim higher."
-- said to me by my 11 year old daughter before going out for hogs 8/13/2011