A merger would allow customers to save on services through wireless, wireline and television bundles AT&T cannot provide because of its limited video footprint, the company said.
The merger will also face an antitrust review at the U.S. Department of Justice, where critics are expected to raise questions about the areas where AT&T's and DirecTV's TV services overlap.
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In Wednesday's filing, AT&T said that DirecTV cannot offer bundled services, while AT&T's video subscribers predominantly choose bundled services, meaning if they abandon AT&T they would be much likelier to switch to cable competitors.
AT&T said it needed DirecTV's customer base to give it scale to compete with its "principal competitors," Time Warner Cable and Comcast. (Disclosure: Comcast is the owner of NBCUniversal, the parent company of CNBC and CNBC.com.)
Comcast's $45.2 billion bid for Time Warner Cable, which regulators are currently reviewing, would put AT&T at a further disadvantage, it said in the filing.