Disclamer
I mostly draw on comment about jQuery in enterprise environment and since I lack experience in YUI, I can not give any conscious advice for [not] using it over Closure.
But in lack of any other answers I'll share my experience with Closure.
Closure library
As for Closure library, which I have been using for last few projects but am, by no means, expert at it, I can say only good things.
Library provides the core components you need when building any kind of UI. But, unlike jQuery, it does not come with trillions of "ready-to-deploy" plugin-in scripts, or as some would say, with no batteries included.
It's got basic events, controls, xhr, dialogs, form components etc., and by my account the most important thing, namespaces (or at least something looking like them...).
With this you can create your own custom UIs limited only by your imagination and the power of JavaScript (and JS is very powerful language even if it does have its own annoyances).
And with help of Closure compiler, which not only minifies the code but it excludes all unused code, does type checking, gives warnings useful for debuging and so forth, it looks like solid foundation for building large applications ground up by teams of any size.
In my opinion, main reason for using Closure over jQuery in enterprise projects is consistency. Plugins are awsome but they tend to include inconsistency at all levels, either programming practices, visual styles and structure, performance, usage, you name it. Removing these small inconsistencies on large project can waste lot of time.
So in conclusion, if you have large project needing custom UI and a lot of flexibility Closure is the Right tool for the job. And with "namespaces" it even feels all Pythonish.
P.S. We also use Django on server side.