MetroPCS was the first to offer an LTE network and an LTE phone, and, despite its limited footprint, that must count for something in DT's eyes. The main reason, however, might be MetroPCS's 9 million customers, which will limit the outflow damage that T-Mobile sustained in Q4 2011, for example, after it became clear it won't be carrying the iPhone.
T-Mobile is the only one of the big four US carriers whose eventual LTE network is not already launched or about to be operational, but Metro PCS' AWS spectrum is not really that valuable for it, since it doesn't hold much to begin with. Moreover, MetroPCS is a CDMA carrier, incompatible with the GSM tech of T-Mobile, and its LTE service is pretty slow, compared to Verizon or AT&T.
Deutsche Telekom is looking at it from an entirely business perspective, though - after the failed $39 billion merger with AT&T, they want to bulk up T-Mobile, make it more attractive for investors, and spin it off in an IPO, or in an outright sale, analysts say.