David - I feel he has a weak grasp on many fundamentals of modern technology[1]. His grammar and spelling are the subject of widespread mockery[2].
I must respectfully disagree with your conclusion that he has a strong grasp of the industry, market forces, or direction. I have a good deal of respect for both Arrington and Malik, and I cannot say I have one iota of the same for Siegler.
He is a writer, and he is paid to write. But the same can be said for the people producing articles for Demand Media. MG's prolific rate of content production doesn't inform its level of quality.
[1] A trivial example is this article: http://techcrunch.com/2009/11/17/twitter-just-ui-puked-on-my... where MG fails to differentiate between Twitter's CSS files failing to load and new features. I could find more if you'd like, but this is one of the more egregious from recent memory.
[2] http://plankhead.com/blog/1146/mg-siegler-destroys-the-engli...