Fantastic article Chris. I am glad there are at least a few of us out there that aren't blind to this fundamental center-channel flaw.
I would love to see this test procedure get brought up during all your standard speaker reviews. Maybe if manufactures saw that people actually do care about the quality of the sound of their center channels off-axis, they would start making a better effort to come up with better designs. Actually, I know of one speaker designer that designed around this problem... Richard Vandersteen... and I am amazed that Audioholics has not done any reviews of his speakers!
It seems to me, that for a site that emphasizes performance and real-world value, that you guys would be big fans of his designs! Getting back to the topic at hand, his center-channel designs are all co-axial, avoiding any kind of lobing and other distortions common in horizontal center channels. Look them up. And no, I am not affiliated, just a big fan. Oh, and ignore the terribly bad, and outdated website, I promise, it does not reflect on the quality of their speakers!