I'm not sure about the reason of the "+2 stops" you can hear everywhere.
Doug explained that the need is "well exposed" picture. That's a thing I would understand, too.
But then you have to have a look first on your calibrator-tool - and take some pictures like here:
Universal Lens Calibrator
I don't use mostly the Phase One Set - I mostly use a "normal" milk glass - a kind of plexiglas (not sure about the real word in english); 2mm strong.
With this I had the advantage, that it's very cheap and I could buy it in different sizes (only 1,- € per piece) - and I don't lose too much light.
I tested it and it must be round about 0.5 stop. So the biggest advantage for me is, that I don't need to extend my time for taking my LCC-picture.
Sometimes I have to double the time - but mostly in direct sun or any similar situation. Otherwise not - and that's a big advantage.
If you have to change your time from 1/125 to 1/30 - it's not a real problem; very simple on the tech-cam, too. But if you have to go from 2 sec. to 8 sec. it's not fun and eats much time, if you want to make a LCC-picture for each picture.
Perhaps with newer 'high-Iso-Backs' you don't have the need for such long times - but I have a Leaf-Aptus with Iso25 and in 'not too good weather' you have to use long time.