That's debatable, though I would not be surprised if Marisa told Reimu at some point regardless of what we do. I think there is benefits in how restrictive it is; we can't exactly start anything without giving ourselves away immediately, and our resources are small enough (even at full capacity) that we wouldn't get very far anyway. With those in mind, I don't mind too much letting Reimu in on it in a controlled, experimental setting, as long as it is accompanied by promises to behave and a reminder that she'll just smash our face in again if we go rogue.