How much does that really matter? I mean, if that's really the only thing standing in our way, then how much objection would there be to a hypothetical re-typing of Necturna as Ghost/Grass instead of Grass/Ghost? I mean, it's not like it would actually change anything about her. How much does precedent matter, though? I mean, if something makes good enough sense that we don't think it's outlandish that the Pokemon developers themselves would include such a feature, then why worry about it? It's not that it would "turn into" a plant. It's more likely that it possesses a plant, which makes a lot more sense. Off of the top of my head, the Pokedex seems to imply that Shuppet turns into Banette by possessing a doll? Though just by looking at the Pokedex entries, one could easily mistake Banette for a standalone Pokemon as well. My thinking on a theoretical pure-Ghost pre-evolution of Necturna would be that it's simply a spirit, possibly a nature spirit, that lives between the world of the living and the world of the dead, maybe sort of like a messenger spirit because that would flow well into the medium concept later on. To evolve into Necturna, it would possess a recently deceased plant. It might do this to gain more influence in the world of the living, but in doing so it might give up some of the influence that it had in the world of the dead.