Rolling your own cellular network would be really useful for custom applications involving sensor arrays out in the world. I have worked on a few noise mapping projects and having sensors communicate to each other while placed out in the world is usually difficult. Controlling the full GSM network would mean that you could potentially customize the way that your sensors communication with each other and with the base station. This could improve the efficiency of data collection.
My series of projects called Sonome were originally envisioned as standalone sensor modules that could either connect to the Internet using cellular data or run through wireless radios back to a base station that had an Internet connection. I think a custom GSM setup would be really great, except for the large startup budget necessary. I thought using XBees was expensive but the lowest-cost GSM base stations are around $700. However, for a project with a larger budget, I would definitely consider it.