All plants are with active phyto-chemicals means having bio activity, in this sense all plants are medicinal. Here medicinal should be taken in broad sense (pharmaceutical, Neutraceutical and Cosmeceuticals).
Yes I think you are true...if all plants are medicinal, then there should be none regarded as non-medicinal. Therefore, the classification as 'medicinal' is unnecessary. However, some may be 'more medicinal' than others and moreso, the medicinal properties and activities of some plants are yet to be fully discovered.
Thanks, Yes your answer is correct, we may classify as "Known Medicinal" (based on validation) and Medicinal (where validation yet to be done or in process).