The entire point to "Looter" is in fact to have a simple rule for when to use Ruins. I had considered this for Curse, but decided to always have Curses out so as not to have to scream at people "you could buy one for your Ambassador" etc. by putting the type on such cards. That's not an issue for Ruins because you've got Curses already.
I understand your logic, especially about curses. But here's what I don't get: you treat spoils exactly the way I was saying you could treat ruins. You don't have a special type dedicated to spoils. You just have a line in the rules saying:
"the Spoils card is not part of the Supply and in [sic] only included when the Bandit Camp, Marauder, or Pillage cards are in the game"
I understand that there is a small but important difference between spoils and ruins: when ruins are in the game, they are part of the supply and can be bought and gained through a variety of means, whereas spoils are not in the supply and can't be bought or gained normally. Still, a line similar to the one above would work for ruins. Granted, it's a slightly more complicated rule than the current "if looters, then include ruins" rule, but the benefit is that we don't have a new card type which is only used during the setup phase.
I guess I'm just saying that in my opinion, every card type should have at least one (possible) reason for existence during the gameplay. Each new card type adds complexity. That complexity is fine if the new type creates fun new in-game interactions (e.g. knights, vagrant/shelters), but the looter type has no effect during the game. Because of that, I think that the extra rule in the rulebook would be a better compromise than the extra type on the card.
Anyway, it doesn't matter at this point. I think it would be neat if Guilds included some reference to looters, but that might violate an unwritten rule about mentioning mechanics which are specific to one expansion on cards from a different expansion.
