That's entirely untrue. Do you really think they go in and build new towers? NO! They piggyback on existing ones.

It's much more cost effective for Sprint, T-Mobile, Metro and the like to use roaming agreements as opposed to building their own native networks outside of major metropolitan areas.

One thing is certain, if you're not going to travel outside of most reasonable sized cities (note I didn't say major) often, Metro's new coverage map is impressive. If you need to roam on occasion, you do have the option.