They don't care about the platform and technology, though I'm sure there's a few patents that are still worth something, but like most acquisitions of this nature they are buying customers and consumers:
http://expandedramblings.com/index.php/yahoo-statistics/
Though it only represents 2% of the market, 2% of the market is still hundreds of millions of individuals, and billions of page views each month. It's nothing to sneeze at.