Plugins and extensions are different things.
The `navigator.plugins` used for fingerprinting is just for NPAPI/PPAPI stuff like Flash, Java, Silverlight. Just about all browsers are also in the process of deprecating support for them.
(it can also be possible to fingerprint browser extensions, but that is a bit trickier as it's not served up directly by the browser as a handy array)
edit: also your comment seems to imply that plugins are used for websites trying to do something the browser doesnt support (ex flash) -- while this is sometimes done, most plugins add additional functionality for the user, ex adblockers, usability improvements, etc