I'm increasingly of the opinion they're both detrimental to the latter, and thus the converse of the former. America survived the demise of the Anti-Federalists and the Whigs, and will survive that of the Republicans and Democrats--if it can last that long. If either major party is a requirement for the countrys prosperity we'll all be in big trouble sooner or later, and right now it looks like sooner. That IRV would make America increasingly multi-partisan almost instantly is precisely why both major parties can unite on that when they disagree on nothing else, but it's also the biggest reason why the vast majority of the public disgusted with both parties should embrace it. The major parties stand to lose a great deal, but the public risks nothing and stands to gain a lot at their expense, which is long overdue.