I would agree that was the reason our forefathers devised a defined separation of powers between the federal government and the states, but, Doyle, is it not a little bit of a leap of faith to conclude what I described as the rationale for why the states remained sovereign.
In what I have read, I have not seen "state sovereignty," per se, defined in terms of the states performing on behalf of, or in the place of the federal government - no matter how rationale and appealing that interpretation is.