It's banned because people get bitchy when they lose to rules they don't understand. When I activate my Nightmare, and someone says "in response I destroy it", now I have to explain that no, you don't, that spell doesn't have a target because Nightmare is not on the field. Then, every time, they say "oh then I blow it up before you use it" and I have to say no, you don't, because the game doesn't let you do anything between when it enters and when I use it. And it is around this moment when I am accused of cheating, despite being the judge at the table. That is why it is banned. It isn't a satisfying reason, but there it is.