I tell this to my design team all the time. They often suggest some jQuery plugin (or the jQuery UI library and all its bloat) to solve a simple issue, like “make a fading image slider”. These plugins often come with 10 or 20 different transitions, or different UI elements that we don’t even use, and half the time they interfere with other javascript we have running on the page.

Most of the time, its easier to just write your own. That way, you know what’s in it, and when it breaks, you know how to fix it.