As the person who first brought Ember to Yahoo a year ago, I can tell you that both the mood and perspective towards SPA development has changed significantly; it's refreshing. Developers are definitely beginning to embrace the direction of web components, and the majority now see the value that these newer frameworks provide.
There was a time (not too long ago) where Yahoo mandated the use of YUI. We are now seeing the pendulum swing in the other direction, and teams have more freedom to choose which framework works best for their situation.
Out of all the modern SPA frameworks, Ember is currently leveraged the most right now here at the 'hoo, with almost two dozen projects using it. This is mainly because our division adopted it early, and we were fortunate that our UI engineers were able to get over the learning curb and build some impressive apps pretty quickly. Besides Ember, there are pockets of Backbone and even Angular apps. However, it's pretty clear that the YUI team is especially intrigued with React right now, mainly because it is the most lightweight (again, pendulum) and allows more freedom for the developers to do things their way without opting into a more opinionated framework.
Some on this thread have expressed that they wished Yahoo would have recommended an alternative. Well I can give you my personal answer:
Choose the best framework that fits the job.
Each brings its own strengths and weaknesses, and the best approach you could take is to understand the nature and scope of your project to know which one makes the most sense for your needs. For example, many projects at Yahoo have a ton of code that can't immediately be replaced or refactored. For those projects, React may make more sense because it only solves just one piece of the puzzle (albeit very well), and can add a ton of incremental value. If you are starting a project from scratch, choosing Ember or Angular might be the better choice if you want a more mature framework that addresses the many facets of SPA development. We happened to put more weight behind Ember for our greenfield projects because it provided more structure than Angular, and that helped us immensely when our apps grew in complexity.
It's really great to see the state of JavaScript development in 2014. Even though we are losing a great framework in YUI today, the future does indeed look bright. Cheers!
--Sam