My use-case for the plugin-usage-plugin is to determine which plugins can be removed without impacting jobs. As part of this, I often use it to track down which jobs must be modified before safely removing a plugin.
Job properties, such as from rebuild or throttle-concurrents, are always reported as used. I think this is because they save config in every job, whether that config is enabled or not. This is misleading, because a disabled feature can be safely removed without impacting a job.
Here is a partial example from a FreeStyleProject's config.xml:
I would only expect this to report as used if <throttleEnabled>true</throttleEnabled> were present.