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.
Generally, I find that clients have no emotional attachment to WordPress--they just want a CMS. So they tend to be less forgiving of things like donate buttons and brand-named plugin functionality. In fact, it makes them nervous. They just want to see a button labeled "Contact Form", not "CformsII" or whatever. Plus, enterprise clients have legal departments that are sometimes worried about licensing and ownership of code and all that, and they don't want to have to clear licenses and terms of use for a dozen 3rd party plugins.
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.