While I'm usually wary of giant corporate mergers like this, the fact that they are 3rd and 4th leads me to believe it would actually position them to better compete against Verizon and AT&T in a way that would be good for consumers.
Let them invest to compete, instead of merging. Merging will only make things worse.
If they don’t, Sprint tanks, and we still have three main market leaders, however two will remain significantly ahead of the third. Keeping in mind that the two that are ahead are now playing the monopoly game and buying into other industries (see AT&T and Time Warner or AT&T and DirecTV or Verizon and Yahoo, etc)