In my experience, the time it takes to become proficient with an existing framework is generally a lot shorter than writing a framework yourself. And, generally, the existing one will have been vetted by thousands of people before me.
I use it sparingly. Mostly because there are many quirks between the various browsers and jquery seems to have a pretty good handle on it. As an example: rather than using one of the many bloated pagination libraries I found based on Jquery, I rolled my own in less code. It ended up much smaller because I didn't have to incorporate everything in the existing libraries. Most frameworks throw in everything, because they have to account for all possibilities, rather than just the ones that you need. You end up with bloated code, unless you know how it works behind the scenes (and many times even if you do, you can't take out the parts that you don't need).
Most frameworks do not let you do this and you have to actually have to work at it/hack at it to use functionality that isn't provided by them.