The main example that Stevens cited of it occurring in the U.S. was from 1868. It was disputed whether the other examples actually constituted in-person voter fraud, with the exception of one instance. So basically it boils down to ONE example of in-person voter fraud in modern American history. Clearly that justifies the imposition of laws that would cost tens of millions of dollars to implement and would certainly result in hundreds of thousands -- probably millios -- of people who would otherwise vote not voting. Because there's one documented case of voter impersonation in modern memory, and no guarantee that it might not have happened anyway by virtue of a false ID.
Certainly this shocking evidence explains the Republicans' sudden fervor to reduce voter participation among traditionally Democratic voting citizens.