If this was being rumored 6 months later and the majority of the U.S.A. will have had LTE available through Sprint, I would be more open to this.
If both companies were working toward VoLTE (Voice over LTE) in the near future, I'd be more positive about it.
I know that Deutsche Telekom wants to sell their stake in T-Mobile U.S.A. but if the combined company will be fighting a plan to move all customers to GSM or CDMA technologies, forget it. GSM is older than CDMA, and much less secure, regardless of being deployed in many countries. CDMA can do concurrent voice and data, but Sprint doesn't seem to be able to deploy the technology consistently, so it's not useful.
Why Dish Network isn't buying T-Mobile U.S.A. is a good question. They tried to buy ClearWire while Sprint was trying to buy it.
The only reason I can fathom that Sprint has to buy T-Mobile is from the executive view--post paid subscribers. The combined company would have fewer subscribers than AT&T or Verizon but come fairly close. However, as executives don't think often, they would be missing the pain of making people switch--something that should be a very strong memory from the 2006 merger with Nextel and their incompatible network, which was just decommissioned this June.
Given all the crap of the last 7 years where Sprint went from a data powerhouse to a powerhouse of shortages, why would anyone want to chance repeating that?
As a long time Sprint subscriber, I don't want to repeat the mess because VoLTE is too far away, and I'm not switching to GSM, and I'm sure that T-Mobile subscribers aren't switching to CDMA, and T-Mobile already has the MetroPCS conversion underway.
Leave things alone until 2016, Sprint and T-Mobile, and I'm okay with it, but right now, no thanks.
Update 2014.01.03: I've heard very little about this since the initial rumors. Softbank/Son Masayoshi is interested in expanding in the U.S.A. Having two brands and their customers would certainly be a sizable presence. Making the rest of it work would be difficult. I'd hate to see Dish Network buy T-Mobile, though. AT&T/SBC seem to be putting pressure on T-Mobile with a new incentive plan. Those who left AT&T/SBC for T-Mobile probably won't return unless prices are reduced and contracts are ended, though. AT&T/SBC seem like the perfect 1950s type of company. They do everything but innovate--confuse, intimidate, complain.