Personally, and this is more of a general coding thing than specific to web development, I find a number of imported items are easy to code and write myself, and thus I have control over debugging rather than submitting myself to the process of reporting a bug then hoping it will be fixed in a timely manner. I also have feature control and can avoid feature bloat, and I have full knowledge of the code I am running.

This allows me to speed up development and have a streamlined custom implementation of that feature for my code. For example, I was unhappy recently with performance and feature set in RRDTool. Seeing that updates from the dev for that are quite glacial I wrote my own implementation, which took about 20 minutes, and now I have one less import, and in the process speeded up data retrieval, so I can have 60fps graph generation, discarded unused features, and added a couple of custom ones for my application.