1. Absence of evidence is not evidence of absence.
2. I buy diary products produced by non-rBST treated cows not because I am concerned about optimizing the nutritional profile of the dairy I consume. I only buy diary products produced by non-rBST treated cows because I want to support animal husbandry practices which do not see cows as machines to be tuned-up with injections. I want to support dairy farms which let cows be cows.
It may be that there's no difference in the milk, it may be that we haven't discovered it yet. But just as with organic fruit, vegetables, and grains, my concern is for the whole. I want to support farmers who do not poison the earth or abuse animals. The food grown conventionally may be just as nutritious, that doesn't matter.