Actually it isn't valid. As I've said. The services those MVNOs provide do roam, and do support LTE roaming from other companies. Just because you use your home/location as something special doesn't mean that it is. Spring and AT&T consider your area NOT worth doing business in. Private companies have that choice in how they operate. And they are a true national carrier. They operate in more than one state and cover more than one state/area with service, thus making them a national provider. Comcast is also a national provider of services and Cox. Why? They do business in more than one state.
And cover means where they have native coverage. Look at VZW's map, why don't you enlarge that in many areas and find out how much of it roams onto other networks. The same as with AT&T. And you do know how VZW got to the size they are now right? They NEVER built out that network they have. THEY BOUGHT IT. Sprint BUILT their PCS network completely out and from the ground up. Can't say that about VZW and AT&T who also bought most of their network.