I don't find that argument unpersuasive; after all, we expect art and other vicarious representations to shape and motivate us in other contexts, so why wouldn't porn have the same capacity? (And I always love it when a group like Reagan's commission winds up agreeing with radical feminists like Catherine MacKinnon.)
Still, the odds of establishing a conclusive link seem low. Even in cases where a sex crime is perpetrated by a consumer of porn, surely the difficulty is in being able to document the difference between causation and correlation.