Reading the article, and realizing that these are probably the best arguments against A/B tests that designers can offer, has actually made me change my opinion in favor of A/B testing. I'll try to go for a point-by-point reply:

    > A/B testing may only be as effective as the designs being tested

True, but taken on face value, these words mean that A/B testing becomes more valuable when you have great designs made by great designers. Presumably not what the author wanted to say.

    > As is true with any usability test, you gotta question
      the motives behind the participants’ answers/reactions.

True, but how does the designer know in advance that "questioning the motives" and "applying healthy skepticism" will yield conclusions favorable to the designer?

    > A/B testing is typically used for tightly focused 
      comparisons of granular elements of an interface, 
      resulting in poor pastiches with results drawn from 
      different tests.

This point is correct, as far as I can see. It's actually the strongest argument against A/B tests that I know.

    > Would you A/B test the Wii or Dyson or Prius or iPhone? Against what?

This point doesn't have any bearing on whether you should A/B test websites.

    > A/B testing locks you into just two comparative 
     options, an exclusively binary (and thus limited) 
     way of thinking.

This point is wrong. A couple months ago we ran an A/B test with four alternatives.

    > A designer (and team) should have an opinion at the 
      table and be willing to defend it, not simply cave 
      into a simplistic math test for interfaces.

This point only asserts what the author is trying to prove.