With hundreds of millions of Facebook and Google users, it makes sense for every company, even an obvious competitor to include this access point. It’s also great for the user, as its one less username and password to remember.
A quote from their blog: From Yahoo!’s perspective, any signed-in user engaging with Yahoo! services is a valued user, whether she authenticates using a Yahoo!, Facebook, or Google ID.
Users will be able to log-in via Facebook or Google and comment on a Yahoo! News articles, play Yahoo! Fantasy Sports, or interact with photos on Flickr, including Flickr uploader. Yahoo! first introduced OpenID log-in with Google on Flickr in October 2010, and following the success of this integration, they’ve chosen to open it up across all Yahoo! platforms.
To initiate their new “one-click” sign-in simply click on the “Sign in with: Facebook or Google” icon on the Yahoo! login screen. You are then fully signed into Yahoo!.
However, a word to the wise- you still won’t be able to sign into a mobile or desktop client using Google OpenID.