Well, y'know, if you have progressive values, and elect a legislator from a progressive third party, he's going to tend to caucus with the more progressive of the two big parties, and cast his vote for the progressive bills that come up -- so he'll still be voting the way you want him to, and is MORE likely to do that than a more "centrist" candidate from the less-regressive mainstream party.

Meanwhile, in a Parliamentary system, a candidate from a minor party will be equally meaningless unless they, too, enter a coalition with one of the Big Guns.