“But it’s clear what advertisers want,” he said. “They want to combine the data intensity of internet advertising with the clear value and ability to change peoples’ perceptions that you get with a television ad.”

Others see great potential for advertising in the merger, which would marry AT&T’s more than 100 million subscribers across its wireless, broadband and DirecTV properties, with Time Warner’s offerings, which include CNN, TBS, Cartoon Network, the website Bleacher Report, HBO and Warner Bros. Pictures. On Tuesday at WSJD, a technology conference organized by The Wall Street Journal, Mr. Stephenson said that, starting next month, the company would offer a 100-channel internet-based premium television service that would cost $35 a month and allow people to watch video on their mobile phones and other devices.

Rob Norman, chief digital officer of WPP’s GroupM, a major media investment group, said the success of targeted advertising would hinge on cost. He offered the example of a home improvement business advertising grass seed in places like Arizona and Montana, which have significantly different climates. That advertising could be further tailored for whether viewers lived in a condominium versus a suburban home with a lawn, he said.

“If you believe in a future where the very, very fine targeting of households or individuals with specific messaging makes economic sense to do at scale, what this merger does is enable that by making more audience available to target in that way,” Mr. Norman said. “The question will be: What is the premium for addressability for that level of targeting that the advertiser is willing to pay?”

Even as DirecTV, purchased by AT&T for nearly $50 billion last year, and others have sought to improve targeting and analytics on cable in recent years, it has remained a “relatively patchwork” operation that is often time-consuming and inefficient, said Tim Hanlon, founder and chief executive of the Vertere Group, a media consulting firm. “It’s probably the last major medium to undergo digitization and sophistication to target audiences in a refined kind of way.”

The biggest change is that television “has evolved to being more fluid and digital-like,” as it appears on devices like phones and tablets and streams through accessories like Roku and Apple TV, Mr. Hanlon said.

“This is clearly one of the significant envisioned benefits of this tie-up, which is the next generation of video advertising in a much more robust, targetable and data-rich manner,” Mr. Hanlon said. “It’s almost as if the realm of digital advertising has been a dress rehearsal for the medium of television, which is now essentially getting ready for its full-fledged digital transformation.”