Indeed, this would be useful.

Also useful would be the ability to peg the view quality of a given layer to be no greater than a given value (so you can keep some layers in outline) and (one which I've wanted for ages) to be able to turn an entire layer semitransparent so you can accurately draw beneath it (yes I know you can group the whole layer, set a group transparency, do your drawing, then remove the group transparency, but it's tiresome).