The DOJ is suing to stop the AT&T acquisition of T-Mobile. AT&T and T-Mobile have been running ads promoting their positions on the merger and the advantages to mobile customers of both companies. AT&T customers in particular will benefit from T-Mobile's excess system capacity. T-Mobile will benefit by receiving value for that capacity.
I don't think AT&T has any chance to win this dispute. During the historic DOJ v Microsoft dispute, I interviewed a DOJ representative and was told the primary criteria for pursuit of an anti-trust case was the numbers. The number of competitors in a market before the proposed change and the number afterwards. If the change reduced the number of competitors, then a lawsuit would be pursued. Period.
I guess AT&T and T-Mobile made the same mistake Gibson Guitars did - not enough campaign contributions to the appropriate people and not enough support for Unions. At least they haven't been raided - yet.