It would have made much more sense for T-Mobile to merge with AT&T given the GSM heritage of both wireless networks. This is one situation where the sum of the whole is probably less than the parts. Bleh!
-source: run a large MVNO.
Remember- MVNOs exist solely to sell excess capacity, and the contracted rate for service is often renewed ever 3-5 years. This merger provides an opportunity to accelerate that review.