I evalutated JavaScript libraries for my current employer and obviously had jQuery and YUI as two of first on the list before I even did any real research.

For me, YUI ended up being too much like a solution architected and run by people coming "down" to JavaScript from more high level languages, whereas jQuery still works like a framework built by people who use JavaScript a lot and wanted a way to make their lives easier (though granted the jQ UI stuff is a bit off the mark). Immediately even the way the YUI documentation hub is written and managed put some of our more designery-developers off and once they felt intimidated by that then their next reaction is to come up with reasons why they don't want to use it, or other developers end up wasting time supporting them.

I thought YUI would make a more suitable choice for us in terms of the approach the YUI team expect YUI code to be architected and written, but if the entire team can't get behind it then rather go with something less architected and at least have everyone on board from the start.

jQuery is popular because it gave a solution to "I was always told to avoid JavaScript but now everyone is using it, how do I get into it easily?". YUI solves the slightly different issue of "I want to base my entire codebase off a well architected JavaScript core that I can extend in a way that my OOP front end engineers will feel at home with".

Sabre wrattling not required.