And so the "mystery" rule proves to be a one-time, save-someone's-ass rule. I don't have any problem with it. After reading rumors that this indeed is what they had planned, my one concern was that they would be able to exercise it to many rounds into the competition.
Hearing Simon say it must be invoked prior to the Top 5 seems a reasonable way to do it. Allowing it any deeper into the competition would allow them to play to large a role in shaping the final weeks of the show. This way, for the most part, they'll just have to stick to pushing certain singers with their post-performance comments.
When they do use it, whatever ramifications it proves to have on the competition will likely stir controversy one way or another. Of course, that's sort of the point. But I think it's an acceptable, minimally invasive tweak to the system.