Yes, I do get that. I agree it would make it easier to edit, or to give them more room to edit. It just seems, unlike with live action, is an unnecessary luxury that, in terms of the final product, wouldn't necessarily yield any better results. Hell, it's not even necessary in live action. Didn't Hitchcock plan and shoot only exactly what footage he would need in order to make it impossible for the studio to come in and cut it any other way than how he intended?

Anyway, to sum up, I think it's an interesting way to do it, but it seems like the costs far outweigh the benefits, and I can certainly understand why it never caught on as a method of doing special effects.