This is technically true, as it is a Layout Option, it is fixed for that Layout. But you don’t have to use those margin settings at all if you don’t want to! If you need that sort of margin flexibility, just set them all to 0 and create Page Templates that give you your desired layout.
With margins set to 0, I could have a first page template like this:
… or like this:
Or whatever type of layout I want, and switch between them with a single click. If you want complete flexibility, don’t bother with the margins and design as many Page Templates as you need to suit your requirements.