Why spend time re-inventing the wheel? If you're looking to sharpen your skills, then go for it. I believe most people would prefer to get their product out the door as quickly as it's reasonable.
It's up to the dev to go through source and understand it. Having a test suite helps.
If you are making a custom implementation specific to your application, most of the css and js is not needed (especially if you are using jQuery) or are useless and need to be changed. It would be so trivial to write you would not even think of finding a library for it.