[noirbot]

This is actually a thing for US Congress races in a number of states, including Georgia and California, I believe. The issue that comes up is that it consistently rewards a stronger party - for instance, if the population is, let's say, 30% hardcore party A, 30% hardcore party B, and 40% who could go either way, what often happens is if party A runs 2 candidates only, and party B runs 5 with a diversity of ideas, party B could end up winning 60% of the vote overall, but at 12% per candidate, and party A could win just 40% spread across 2 candidates, but have both of their candidates easily make the runoff.

It may be better in a world where there's 5+ real parties who will never back down from each other, but even in that case, it seems like it rewards parliamentary-style coalitions where if you're polling at 5% or so, you just tell your supporters to support a stronger candidate you mostly agree with so that you don't get locked out of the final vote entirely, and you're back to essentially either a two-party system, or a two-party system with minor parties that mostly play spoiler to the main party by pulling off votes.

One of the biggest issues I tend to find with a lot of voting proposals is that they seem to assume that the parties and candidates will act only to serve their party/candidate directly, and not strategically run/withdraw in order to help/hurt other candidates, or collude with each other to game the system.