Any benefit in coverage area would be a long term proposition. T Mobile and Sprint use incompatible technology. Sprint CDMA phones will not work on T Mo GSM towers and vice versa.
Sprint's coverage is dismal outside major cities and along Interstates. T Mobile is much better now than Sprint as far as coverage area, reliability, and data throughput.
IF the merger goes through, and I think it's fairly unlikely, the combined company would most likely phase out CDMA over a period of years and replace it with GSM technology or probably more likely phase out both and go LTE only with no 3g/2g whatsoever.
I'm uncertain as to whether the deal would benefit T Mobile's bottom line. Sprint is very nearly bankrupt as it is, and would bring very little value to the deal other than some additional frequency bandwidth which T Mobile could use to improve their already superior service. T Mobile is profitable, but not at the same level with AT&T or Verizon.
I spent time on Sprint. The native coverage is tiny. They roam onto Verizon and others which have much better coverage area, but you generally get talk and text only in roaming areas - no data.
AT&T and Verizon have much better coverage areas, but T Mobile is not that far behind and is expanding quickly.
