It actually makes a lot of sense. JQuery is a pleasure to use for general UI manipulation, but (to my mind) it just doesn't have the consistency and quality of

It actually makes a lot of sense. JQuery is a pleasure to use for general UI manipulation, but (to my mind) it just doesn't have the consistency and quality of YUI when it comes to UI widgets / framework. And honestly, it will always be hard for a framework developed as plugins by a diverse group of people to compete in this regard with a full stack framework where every single widget is developed by a crowd of people under the same roof - you get the same look and feel to exacting standards with features most of us don't even want to think about built in (eg: ARIA etc.). Why not use YUI for doing general ajax work? Well, it's API is horribly verbose, to borrow a term from Zed Shaw, it's "autistic". I can't imagine why they really thought it was a good idea to make it that way but they did. So combining the two makes a lot of sense to me.