Except then, no one else on your team has any idea what is going on and spends hours making minor changes. And it's probably not tested because jQuery is generally quite a bit harder to test so everyone is scared of making changes.
One time I directed an intern to implement an in-browser list filter functionality using an existing open source library. When I saw his work, he didn't use it. Indicated that he didn't fully understand how to use it. He wrote his own implementation using jquery. It technically worked for the few test cases we tried. It was a mess to look at, with Jquery hooks all over the place. Argh..
