Websites can still check whether a particular hidden plugin is installed by directly querying navigator.plugins like navigator.plugins["Silverlight Plug-In"].
This code change will reduce browser uniqueness by "cloaking" uncommon plugin names from navigator.plugins enumeration.
If a website does not use the "Adobe Acrobat NPAPI Plug-in, Version 11.0.02" plugin, why does it need to know that the "Adobe Acrobat NPAPI Plug-in, Version 11.0.02" plugin is installed?
If a website does need to know whether the plugin is installed or meets minimum version requirements, it can still check navigator.plugins["Adobe Acrobat NPAPI Plug-in, Version 11.0.02"] or navigator.mimeTypes["application/vnd.fdf"].enabledPlugin (to workaround problem plugins that short-sightedly include version numbers in their names, thus allow only individual plugin versions to be queried).
The default pref cloaks all plugin names except Flash, Shockwave (Director), Java, and QuickTime.
Things like geolocation, database access and such require the user to grant permission for a given site.