I don't doubt that papyri should be rare, but that raises the question of papyrus and papyruses, both currently classed as common. I think the singular word is reasonably well-known, and can remain as common. However the alternative plural form, papyruses, is actually used much less often than papyri (and is queried by my spell checker, unlike papyri.). I suppose the thing is that we mainly think of papyrus as meaning a material - a word that doesn't take a plural. Papyrus meaning a scroll or document on that material is a more technical usage of the word. So I'll reclassify both papyri and papyruses to rare status.
