·Time Warner Cable
·Verizon Online DSL
A Sprint + T-Mobile merger would turn what's currently a "bigger two plus smaller two" market into a "big three" market, with the third member of the trifecta having quite a bit more spectrum (thanks to Clearwire) than the other two.
As opposed to AT&T plus T-Mobile, which would've given rise to one giant carrier, one slightly less giant carrier, and one comparatively small, weak carrier, which definitely wouldn't have been healthy for the wireless industry.
The downside of a Sprint T-Mobile merger is that the pace of rural carrier consolidation would increase. US Cellular would probably fall in Verizon's lap, and CSpire would either end up belonging to Sprint or Verizon. Viaero would go to AT&T. The currently GSM carrier west of me, West Central Wireless, would probably go to AT&T as well.
On the one hand, these acquisitions would strengthen the footprints of these carriers. On the other, the better customer service of some of these local outfits would fall by the wayside as they're acquired by the big guys.