"Tonight we are launching a full post rss reader in the new Yahoo Mail beta. If you are in the beta, youÄôll automatically get the new features.
RSS in mail makes perfect sense for a few reasons: 1) people already spend a lot of time in their Mail experience, why shouldnÄôt personally relevant content be there too 2) While you read RSS you are probably gonna want to forward good stuff you find 3) Hundreds of millions of users use Yahoo Mail, so if we want to reach the masses, we need to go where they are."
That third point is the biggie - millions more people use Yahoo! Mail than use the MyYahoo portal. And most of them probably haven't been directly exposed to RSS feeds yet.