I think the main argument against it was that an extreme candidate could take votes away from a moderate candidate, and that moderate candidate's votes would go to the moderate candidate in the other party, while the first moderate candidate would have won in the first place. As a whole, I think the opened up opportunity for a candidate with wide appeal is worth the relatively small risk of that happening.I would endorse this system - but when something similar was proposed in Britain, a lot of pundits thought it'd be too complicated for voters.