Tools are employed in support of a goal to achieve something. For example: A hammer is interesting, but it´s worthless unless used as a means to hang a picture, build a tree house, etc. You never use a hammer just because you can; you use a hammer to accomplish something greater and more important. Blogs should be employed as a tool in the same way. Blogs are a means, not an end.
Blogs support a company´s on-line strategy to sell or support a product or service. If your key audience isn´t on-line today, employing a blog isn´t going to change that. There are exceptions, but the exceptions require other changes in your business and web strategy.
People should ask themselves one question when trying to decide their web strategy and the decision to blog: "Should my business be on the web and if so, for what purpose?"?? From there, a blog either makes sense or not. Blogs trail web strategy, they don´t lead it.
Blogs — just because you can, doesn´t mean you should.