I have gone back and forth between ball swivel and no ball swivel on the 2nd stage, especially when experimenting with different hose lengths and mouthpieces. I have finally decided to stay with the ball swivel because I can position the hose at any angle I want to. Having the hose angled down seems to work the best. Several of my fellow dive buddies/instructors also use the same set up.
The biggest difference I noticed in reduced jaw fatigue was when I switched over to the comfo-bite mouthpiece. Between the swivel and comfo-bite, jaw fatigue is never a problem any more. The top of the mouthpiece simply sticks to your upper jaw behind your front teeth, eliminating the need to bite down on the traditional teeth tabs.
This combination is especially practical in the realm of recreational diving, but I can see where highly technical divers could argue about having one more point of potential failure, or just something else to have to keep an eye on. Personally, I plan on keeping the ball swivel because it works very well for me.
One potential minimal side benefit of the ball swivel is that it makes the entire primary 2nd stage slightly negative. This actually helps to reduce jaw fatigue by stabilizing the 2nd stage. If you have to do a regulator recovery, especially in swift current, chances are you will find the 2nd stage hanging straight down beside you instead of drifting off somewhere. Mine hangs straight down by my side between my right arm and rib cage. Very easy to find that way. Hope this helps some.