"And it does make a difference functionally." So damn the thematics but what about function? There are tons of combos that are different just because it has that Victory label on it. You say "0 VP" could be extrapolated to the other cards, but you can't extrapolate that "Victory" type, and since it has that type it needs something on the card to say how many VP it's worth. That's not an inconsistency -- it's a consistency. What would be an inconsistency would be if something called "Overgrown Estate" weren't a Victory card in the first place.
Why could I not put 0 VP on all non-Victory cards and have them be a X-Victory hybrid? Why can I not extrapolate the "Victory" type? It doesn't need the subtype Victory in the first place. It is an inconsistency. It would be inconsistent for an "Overgrown Estate" to not be a Victory card. Why do you think I suggested that the name could have been changed to something else in addition to dropping the "Victory" type.
And, to Donald, I agree that it is merely a convention. However, I am of the belief that the convention should be a straight rule, not one to be broken. If it is to be broken, not in this way by the Overgrown Estate. A Victory card should have something to do with VP's, yet this one does not. It defeats the purpose of the subtype Victory if it has nothing relevant to VP's on it.
I don't understand why you're ignoring the functional ramifications, which is the question I put to you in my initial reply.
Why can't you make everything an X-Victory hybrid worth 0 VP? Because then Scout is a triple-Laboratory (an unnecessary buff for what is already a power card! Zing!).
Card types aren't flavor.
Young Nick - the basic types (Action, Treasure, Victory) all mean basic things about the cards they're on. An Action uses up an Action, a Treasure is played during your buy phase, and a Victory is of importance at the end of the game. Different cards reference these types - Adventurer seeks Treasures, Scout draws Victory cards. If you want a card to perform something during your turn, it has to be Action or Treasure - if you want to limit how much it can be played, make it Action, if you want to be able to just throw an infinite amount down, make it Treasure. So Nobles and Harem, while playable, also give VP, so they're combo-types.
As for Overgrown Estate - the thought process was to have a card that cannot be played - a card that just sits in your deck. It also does not yield any VP. But we don't have a card type for that. So make it a Victory - now it can interact with Scout and Silk Road, but that's ok.