Your entire argument is basically "current rules require it for preserving on appeal". Sure. If the FRCP/FRAP/etc didn't require that, can you think of another reason?

We aren't talking about opinions, or motions to decide the case of various forms that will produce such opinions, but just every day motions on requests for production or whatever (which is what FISC is dealing with).

I mean, this is in fact, the whole point of things like magistrate judges.