This is just not persuasive - here's a key point from it: "If we put aside Joel's argument and look at the job market, there's incomplete information, but both

This is just not persuasive - here's a key point from it: "If we put aside Joel's argument and look at the job market, there's incomplete information, but both current and prospective employers have incomplete information, and whose information is better varies widely. It's actually quite common for prospective employers to have better information than current employers!" Ok, sure, maybe there are employers who don't know what they've got. But just based on the raw amount of information on either side, all else being equal current employers should have a much better idea of actual performance.