The documentation is unfortunately vague about this, but it does state that NONE plugins "will be chosen last **or not at all**"
We could very well emphasis the other side of the or.
I don't see any reason not to chose them, there's several seemingly useful plugins that have rank none: all Intel MSDK encoder / decoder plugins, VA-API hardware decoders (not the higher level vaapidecodebin), `avenc_aac`, and of course multiple `avmux` plugins which can be useful here, have rank none.
If rank none was meant for plugins not to be used, `gst_element_factory_list_get_elements` should not return them.