Great,the only concerns I could think of were legal, around ownership of frequency ranges abd interference.
Someone mentioned that building a full networks (or even a full base station) is much harder than just apt'ing some software and snapping some cheap SDR. So we spent couple years developing an SDR which is more expensive than your typical hobbyist one (http://umtrx.org) - btw even hardware of the simple version of our SDR is open-source - and then couple years building a base station which complies with all regulations and has open-source in its heart (based on Osmocom obviously). And now we've added a bit of (unfortunately proprietary) glue and we can run fully fledged cellular networks for carriers - mostly in developing countries, as was mentioned before, because demand for simple voice communications is through the roof there. At the same time you can buy one of our stations for your lab and play with it as much as you want (get your test license first though).
So yes, you can't stop progress and technology gets cheaper and more accessible. Which is very exciting.
There must be some widely used ones in developing regions.