Briefly, the proposition that random testing won't really be random is a common-sense proposition that plays out on our streets on a daily basis. To suggest that defences can be raised based on non-randomness because of an officer's history or other evidentiary matters often places a tremendous burden, practically speaking, on indigent and discriminated-against individuals.
So, that's not an answer, and I don't think that it saves the provision. I think the point you make actually points in the opposite direction.