I love that there's such a huge library of 3rd party WP plugins, and I use them often for personal sites and pro-bono work. But I rarely use plugins from the repository for professional development jobs.
So for the most part, I work from a library of basic, white-label plugins I've developed, and customize them for each client. That way, I know all the code inside and out, and can make customizations very easily and cleanly. And I know there isn't a bunch of junk in there that was designed to accommodate edge cases--just what's needed for the job at hand.
However, I definitely LEARN a lot from 3rd party plugins. Often I'll crack open a plugin to see how it works, and then create my own version of it based on what I learned. Then when the need for that functionality arises again, I have something to work with that I built myself, that's free of clutter and attributions, and that is practically invisible to the client.