The advantage was supposedly that they had less friction so traveled a bit faster, charge-for-charge, and also that they eliminated copper fouling.
Both claims were/are sort of true, but the the velocity gains were small and moly fouling turned out to be harder to get rid of than copper fouling, plus apparently damaging the occasional bore.
You'll find a lot of folks that were all gung-ho for the stuff and now hate it. I think there's a lesson in there...