TMobile has been refarming their 2G towers (1900mhz) to be HSPA+, and will soon also refarm their 3G towers (1700mhz) to be LTE-only. So it won't add the 2G bands to the 3g bands, but instead switches 3g over to the current 2G band and replaces the 3G one with LTE (which the Nexus 4 doesn't have). In areas where you get good 1700mhz coverage but poor 1900mhz coverage, you'll soon get poor coverage on a Nexus 4.
+ the higher the mhz, the shorter distance it travels and the poorer it goes through walls.
+ the higher the mhz, the shorter distance it travels and the poorer it goes through walls.