> So it seems as if it's historically normal to struggle with > mm's crop marks.
I have an old "mmt" file on my computer that simply has .de)k .. but I can't remember whether that's the way it was provided or I had perhaps edited it... However, one good reason for replacing the dashes by spaces (instead of completely deleting the line) is that it won't mess up the vertical spacing, if the macro package is designed to rely on it.