Earlier quoted context omitted.
The REAL answer is "a tax efficient proxy for holding Alibaba shares".
Alibaba's market cap is $174B. Yahoo owns 24% of their stock. That's $41B of wealth right there. That's actually more than their own market cap, which is $28B! I imagine the natural duopoly of tech is at work here. Google/Bing is pretty much it for web searching in the US, with no room for whatever engine Yahoo is borrowing this week. Gmail (gmail.com, google business apps, etc)/MS(Outlook/Hotmail, Azure, Exchange 36…
That last trade price times number of outstanding shares -- market cap -- is the only common basis for overall valuation of publicly traded firms (that is, the only basis that is guaranteed, by definition, to be available for every publicly traded firms) does not mean it is a particularly accurate and meaningful measure of the overall value in any useful sense.