YouTube officially entered the professional television business last week with the addition of some full-length shows from CBS, but the world's biggest video site will need more than just one network's programming to be a viable contender for premium ad dollars.

Via an extension of its existing CBS deal, YouTube now carries full episodes of shows such as "MacGyver," "Star Trek" and "Young and the Restless," marking the first time the site has offered videos longer than 10 minutes and with pre- and post-roll ads.

While the library titles aren't likely to drive a massive audience uptick, the move may help YouTube vie more aggressively for brand advertisers with network-skewing sites such as Hulu, NBC.com and ABC.com.

"It's a good and probably necessary move for YouTube to remain competitive with premium content sites over the long term, notwithstanding YouTube's dominant market share," said Greg Sterling, principal with Sterling Market Intelligence. "It's also smart because audiences will be more tolerant of advertising in full-length programming versus clips."

But for this deal to be a game-changer financially, YouTube will need to build out a strong library of vintage TV and other full-length shows, Mr. Sterling said.

That won't be easy, said James McQuivey, an analyst with Forrester Research. "We will see other network content on there, but NBC and Fox are going to ask [YouTube] to embed the Hulu player, like everyone else has," Mr. McQuivey said.

YouTube said it's aiming to land other TV network deals. "We think this is a big market, and we are happy to see consumers and content owners continue to embrace online video," a YouTube spokesperson said.

But even if YouTube adds more network content, consumers may not flock there because many TV shows are already widely available other places online.

"There's little reason for people already on YouTube to interrupt the site's social, clip-focused experience to watch a full-length episode," Mr. McQuivey wrote in a recent blog post. "And if you hit the Web knowing you want a particular TV show, you're as likely to go to its home page as you are to go to YouTube."