I think Moore's documentaries mostly ride on hype and sensationalism (and yellow journalism) using shock to make those that already share his opinions (yes, I do say opinions, because dispite the presentation of an objective and unbiased "documentary", the clips/commentaries are skewed one way or another) watch his documentary to feel stronger about them while those that don't watch his documentaries out of curiosity and a form of defiance. It's a money making scheme no matter how you look at it.
HOWEVER, Moore's documentaries DO contain truths, and if you look past the bias and skew, these truths ARE a very integral part of the success of his films.
I view Moore's documentaries not as a "shockingly biased" work but as a testament to the freedom of speech that all of us enjoy in America. It presents a view which some may share, other's may not, but the fact that this view is expressed at all and we're here discussing it as civilized men (looks at Volourn)... or maybe not... is a strong message in and of itself.