By Kevin Kelleher
October 25, 2011

By Kevin Kelleher, contributor

FORTUNE — If a company stays on the auction block long enough, rumors about its fate will eventually begin to repeat themselves. In 2008, while Yahoo co-founder Jerry Yang insisted the web giant wasn’t for sale, Microsoft emerged as a suitor. Then Google got involved.

Fast forward to 2011 and here’s Jerry Yang again downplaying reports of a Yahoo (YHOO) acquisition. Among the suitors is, again, Microsoft (MSFT), only this time it’s reportedly bankrolling a deal by joint buyers. And as the day follows the dawn, reports followed quickly that Google (GOOG) is also interested in financing private investors looking to buy Yahoo.

There are plenty of reasons a Google-backed deal would not pan out. First, the Wall Street Journal story reporting the possibility was based on a single source — that elusive “person familiar with the matter” — who said that “Google may end up not pursuing a bid.” Then there are the antitrust concerns: the Department of Justice shot down the 2008 search partnership Google wanted to arrange with Yahoo. The government would surely take a close look at any moves backed by Google. And then there are the strategic reasons — or lack of them. Much of Google’s future is focused on social and mobile — and Yahoo is weaker in both.

Still, that doesn’t mean there’s no virtue for Google in saving Yahoo. Here are six possible scenarios in which it might actually be a good idea:

Google is throwing sand in the gears of another deal. The most cited theory is that Google wants to drive up the price for Microsoft or another bidder. A simple phone call by someone familiar with the matter to a reporter is all it would take. Yahoo’s stock rose 4% Monday following the report. The risk in this move is that this slight rise in Yahoo’s share price won’t last long. In other words, this report would at best delay a Microsoft-backed deal. Much more likely is that Google would try to get into the Yahoo-takeover mix to give Microsoft second thoughts — or even drive it away if it’s on the fence. It could also deter smaller third parties from pursuing their own bid at all.

Google wants a bigger presence in display ads. Revenue from display ads are disappointing at Yahoo but they’re growing at Google, which is making $2.5 billion a year from them. Yahoo, however, has deep relationships with the web’s biggest advertisers as well as many popular sites. If putting Google’s money into Yahoo could open up access to Yahoo’s display customers, Google could see more display growth. Such a move would surely set off antitrust alarms.

Yahoo could bring more users to Google+. For most people, Facebook is still the go-to place to socialize with friends online. Google+ has signed up 40 million users in a few months, but that’s only 5% of Facebook’s 800 million active users. What Google needs is access to more active accounts — and Yahoo still has a lot of them. Yahoo, meanwhile, is even further behind Google when it comes to having a social-networking infrastructure. Since social networks rely on big numbers of users, corralling Yahoo members into Google+ would help Google build a strong rival to Facebook.

Google wants a closer look at Yahoo. When investors enter serious takeover talks with a company, they often demand a look at its financials and performance metrics. By backing some private equity investors who might otherwise have trouble raising money, Google can have access to all that data, even if the talks eventually fall through. There may not be much of Yahoo’s data that is crucial for Google to know, but there’s plenty that could be helpful: How is Yahoo’s search partnership with Microsoft really faring? Where is Yahoo’s display business strong, and where is it weak? What insights does Yahoo have in international markets, particularly Asia? What’s more, if Microsoft is involved in a takeover bid, it may have access to this same data. So why shouldn’t Google have access too?

Yahoo offers Google a backdoor to China. Google’s history in China is a rocky one, culminating in the company’s decision to shut down its China operations last year. But one of Yahoo’s strengths is its assets in China, including its 40% stake in Alibaba.

The truly speculative will consider this: If Google owned a stake in Yahoo, it would indirectly own a stake in Alibaba, one of China’s Internet giants. That could offer a way back into a growing market. It could also derail any chance that Alibaba would buy Yahoo. If Jack Ma controlled Yahoo, he could undercut Google on everything from online payment fees to search and display ad rates. Far-fetched? Sure, but coincidentally or not, Google said Monday that China renewed its operating license in the country. And the former head of Google China said on Quora that Alibaba and Ma aren’t suited to manage Yahoo. It’s hard not to read that and think, “but Google is.”

Investing in Yahoo will keep antitrust regulators off Google’s back. Google may be interested in helping Yahoo to thrive to lessen its chance of becoming a search monopoly. Without Yahoo around, Google could become so powerful in search and display ads that antitrust regulators end up doing more harm than a revitalized Yahoo ever could.

There is a precedent for such a move. In 1997, Microsoft invested $150 million in a struggling Apple (AAPL). Ostensibly, the investment was to encourage Apple to install Office and Internet Explorer onto Mac computers, but it also served to keep antitrust regulators at bay. (A side note: Microsoft later sold off its Apple shares, which would be worth several billion dollars today).

Yahoo’s future is unpredictable, so much so it seems like any outcome is possible at this point: A merger with AOL (AOL), a takeover by Jack Ma, a buyout by private equity firms financed in part by Microsoft, or Google, or someone else. But one thing that is growing less likely is the scenario that Jerry Yang wants — an independent Yahoo.

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