One reason so many companies struggle with new media forms such as blogging and evangelism is that their staff is not deeply committed to the company's business. For instance, if I work for a bakery, I may or may not feel it's the greatest bakery in the world, but I should at least enjoy making and/or eating baked goods. I mean one or the other, at least! Unfortunately, few companies check whether job applicants care. Even worse, many business owners have no particular attachment to how they make money. It's hard to generate conversation about a topic that doesn't interest you.
Gapingvoid Blog: Edelman Talk. 2007-Mar-21, by Hugh MacLeod
If corporate blogs work, it's because they help humanify the company. I wrote about this earlier in an article I called "The Porous Membrane". To paraphrase: Ideally, you want the conversation between customers [the external market] to be as identical as the conversation between yourselves [the internal market]. The things that your customer is passionate about, you should also be passionate about. This we call "alignment".