I've found when Magpul went to the MS3, they fixed their adjustment issue and it's silky smooth now. I agree, the earlier ones, not so much. But as I said, I found the Vickers to be not easy to move (of the one and only one I have).

ETA:

I feel like the only people I've seen that do that are MIL guys... I do it as my old partner was a retired 1SG and told me to setup my sling like that and I never changed. So simple, but brilliant.
I started doing it because, as 11B said, it prevents the cheek weld issue while also still giving a flat purchase on your shoulder, plus if you adjust the sling correctly, it allows you to go weak side if needed.