His argument seems reasonable enough to me, and it helped me to understand the different tendencies of Approval and IRV. Let me state what I think they are.
Like plurality, IRV tends to reach a stable equilibrium with two dominant parties. In that equilibrium state the effect of uncompetitive spoilers is eliminated, but minor parties are unlikely to win. If a minor party does somehow manage to defy the odds and become seriously competitive, IRV can end up electing a relatively unpopular candidate (just as plurality can with a strong third-party candidate).
Approval, on the other hand, will not necessarily reach a stable equilibrium with only two dominant parties. If I understand it correctly, it could allow any number of parties to reach a stable state of parity. That could be revolutionary, of course. Imagine the Libertarians and Greens actually competing on an equal footing with the Democrats and Republicans!
But I see a potential problem. If several parties reach parity, strategic voting becomes difficult, and the voter could end up with very difficult decisions. Do Greens approve the Democrat as a hedge against the Republican, or do they not? It's the same old problem, except now it's much more complicated. Now Greens may need to worry about letting the Libertarian get elected if they don't vote for the Democrat. As a matter of fact, they may need to think about approving the *Republican* to prevent the *Libertartian* from getting elected! Or vice versa.
The bottom line is that the election could be tipped in any of several different directions depending on where voters decide to "draw the line" between their approved and disapproved candidates. Maybe this is obvious, but I wonder how many have really thought about the potential consequences. I am not claiming that the disadvantages of Approval outweigh the advantages; I'm merely pointing out the potential problems.
The Approval strategy "formulas" that have been put forth are interesting, but in many cases they will be of no practical use to voters. Why? Because they are based on how other voters are expected to vote, which is obviously uncertain. The best strategic cutoff point could depend critically on that uncertain information, in which case the formulas will be useless as a practical guide for voters.
I understand that Approval is used by several professional societies, so it has been tested to some extent. As far as I know, however, it has never been tested in large-scale partisan public elections, so it is largely untested. Elections for professional societies tend to be very uncontroversial and participation is often poor, with only something 10-20% of the members even bothering to vote in some cases (even though you can vote by mail).
--Russ
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