It's only Alibaba stock they hold that creates any "value" in Yahoo.
"The latter part of the business is clearly pulling the former along. Here’s how to think about it: Yahoo has a stock market value of $34.1 billion, and owns a 24 percent stake in Alibaba. Estimates for the current value of Alibaba, which is planning an IPO, range between $75 billion and $125 billion. If it comes in at the low end of that range, Yahoo’s stake would be worth $18 billion, or just over half of Yahoo’s total market value. At the high end, Yahoo’s stake in Alibaba would be worth $30 billion—which would imply that the rest of Yahoo’s enterprise is worth as little as $4 billion, or 12 percent of its current value."
http://www.bloomberg.com/bw/articles/2013-10-16/how-much-is-...
http://www.marketwatch.com/story/is-yahoo-really-worthless-w...
http://qz.com/528986/charted-the-value-of-yahoos-alibaba-sta...