If you look closely at the ZMA study, you'll see that all subjects were deficient in Zinc and Magnesium before taking ZMA. All increases in natural test output, GH, IGF-1 were achieved by simply correcting the nutritional deficiency.
Bodybuilders are notoriously deficient in these minerals; as the lifting process saps the body of them. In my opinion and experience, ZMA is a very beneficial and affordable supplement.