I just don't see the advantage of the swivel. It never seems to really help with the first stages I have (MK-5, MK-10, & MK-20/25).
On backmount doubles, the long hose would go on the swivel turret (on the right post), pointed down (normally). The use in the swivel is for if/when you donate the long hose. At that point, you can fully deploy the hose and the turret swivels so that the long hose routes straight from the 1st stage over your right shoulder. It does not have to make a 90 degree bend right out of the reg. No chance of pulling on it and kinking the hose (as I have heard has happened to someone who was using a MiFlex hose for their long hose).
If you never donate the long hose, then the swivel really has no benefit (in a BM doubles config).