Recently, we were working with a very large company on a potential blog. After much discussion, the executives decided that a blog was not appropriate for the company. And it was probably the right decision. The company is primarily a one-way communicator, and because of its business, cannot risk (or tolerate) much dissent or negative feedback--particularly in an open forum like a blog.
While I agree that blogging isn't for everyone and may be an inappropriate means of communication for some companies in some businesses the reason given above seems the worst of the lot.
Any business that " cannot risk (or tolerate) much dissent or negative feedback" might as well give up now!