Guess why Graham's Phantom II has now an added, permanent "spirit bubble" to his latest creation...
Good question, Axel. I can't imagine anything less useful (except maybe a Wally Skater).
As Essentialaudio said, getting a headshell level is no guarantee of proper azimuth. It's not even the best visual starting point. Balancing toothpicks on headshells a la VPI or futzing with a Phantom bubble is focusing on the wrong parameter. Assuming the arm isn't flawed, as Psag's appears to be, azimuth is about adjusting the stylus in the groove. That's where you should start - that's where you should finish. Nothing else matters.
Given the multiple variabilities inherent in even the best cartridges, levelling a headshell is foolish. Spending money on a bubble to level a headshell is just foolishness squared.
Perhaps the liquid provides some useful damping? ;-)