What Makes Black Rock City a Great Test Site?
Almost anywhere you go in North America there's probably a least a little cellular coverage. That creates a problem for testing an experimental basestation with live subscribers. Your cell has to advertise a 'network identity'. You can't borrow identity parameters from the pre-existing network since that would risk disrupting service for the existing carrier, and it's probably illegal to masquerade as some carrier's network anyway. (That would make you an IMSI-catcher.) So you have to become a fully licensed common carrier, which is very expensive, or you have to advertise some unclaimed, oddball network identity, which means that all of your test subscribers need custom SIMs, which means that they need unlocked phones. Since most phones in North America are locked, you have to supply custom SIMs and unlocked phones for hundreds or even thousands of test subscribers. That's not hard if you're large and well-funded, but if you're a couple of guys still working out of a garage it can be a big problem.
However, if there is no other cellular coverage at all, you can use any network parameters you want and phones will eventually try to camp to you out of sheer desperation. Your test subscribers can use any SIM, meaning that they can also use the locked phones they probably already own. And if it's your experimental service versus nothing at all, people are more willing to be good sports while you stumble through those first couple of days.
So this is what makes Black Rock City so good: a large number of people in an area with no cellular coverage. In fact, there's no telephone service of any kind. But lots of people brought their AT&T and T-Mobile GSM phones for the drive out there, and those phones will work on your cell, even if they are locked, because there's absolutely no competition.