I've been messing around with different Rogue decks and this to me seems like something that should be refined/reworked slightly to compensate for the other (0) spells. It currently feels like a bug and it limits all Rogue players capability to pull off certain plays.
Rogue is a class that revolves heavily around the order of actions the player makes but this behavior basically adds a speedbump sometimes. No matter what Rogue deck you're playing, if you use Preparation and other (0) spells, you will run into this issue. Considering that there's already the RNG element of the game that all players have to seccum to... You're telling me that if a Rogue decides to use Preparation, Backstab and Shadowstep in their deck they run the risk of running into a scenario where 1 or 2 Preparation cards may go to waste? I think the new Preparation spell should read something like this: "The next (1+) spell you cast this turn costs (3) less" or "The next spell you cast this turn costs (3) less, (0) spells are unaffected".
I can't even begin to count how many times I've attempted to make a combo/strategic play but the current Preparation effect destroys my strategy because I used it before using another (0) spell. I'm sure I'm not the only one that has run into this, I'd love to hear what people have to say about, if whether or not they think is a big deal or not.