Tournament, Island, Monument, Pillage, Madman and Baker all only have their normal types. Sure, you only need to get out VP tokens, coin tokens, Island Mat, Prizes or Spoils if they are used. But you could just as well imagine they always are there. It's inconsequential to the game if you put VP tokens on the game and no +VP card is in the kingdom.
I think the difference between these cards and the Spellcasters, and what made me think of this, is that the Spellcasters add an extra step to setup where you have to somehow pick which Spells are being used in that game. I'm not sure how far you're planning on going with this expansion, but if you were to create a rulebook, it would be much easier to say something like "If any Kingdom card has the type Spellcaster, randomly select 3 Spells to be used in this game" and then whatever suggestions you want about how to select the Spells.
What you say about it not mattering if Spells are available in games without Spellcasters makes sense to us as humans, but Dominion is supposed to run like a computer program. I think without this rule it would imply that if you own this expansion, randomly select 3 Spells for every game and just let them sit there (if that makes any sense). I'm just imagining having this on MF and for every game having 3 random Spells on the next page, whether or not they're doing anything. But maybe I'm taking this too far. Anyway, that's just my thought process for this.
Edit: Okay, another thought just came to me: All of those cards that you mentioned are either the only card that adds its thing to the game or deal with tokens (or both; in the case of Baker it's the only card that gives you a token to start with, but it says that on the card.) With the tokens, it doesn't change the game in any way to have the tokens sitting there. For instance, in my post
here I noticed that one of the bots had a Journey token next to his avatar, even though there was no Journey token card in the game. This, however, didn't change anything about the game, as there wasn't anything he could do with his token. But with Spells, it does actually change the game, however inconsequentially, as it gives you more options (you can buy them), and there's more than one card that deals with them. So I guess that's another part of my reasoning for why there should be a card type to decide when to include Spells.
Okay, I'm done now. This mechanic is really cool, by the way!