People use jQuery + <insert other js frameworks> because they work better, are easier to use, are lightweight, and don't trap you into using the entire monolithic package that is offered by YUI.
i like the internet because it has cats. it's better than phones, which traditionally had no cats.
edit:
I don't mean to come off dickish. I started out writing my own AJAX libs in 2002-2003, started stealing some good ideas from Prototype in 2004-5, eventually trusted my fate with jQ in 2006, and have facepalmed every time anyone recommends we use YUI since the day of its release. It's like the worst of both poles...it is neither a fluid, lightweight swiss-army knife like jQ, nor a well-constructed application framework like ember. Instead it always seemed to be a rigid set of off-the-shelf components that appealed to people who liked to talk about design patterns but not actually build anything.
