No, is the short answer, and correct for many companies, but not all.
Blogging is very much like PR, search engine optimization and journalism. Developing relevant content in all three disciplines will help the writer to achieve their goals. For PR it’s generating goodwill. For SEO, getting noticed by your audience. And for journalists, reporting the verified facts.
Marketing Directors know a lot about marketing, and sometimes know a lot about their product or service. That does not mean the Marketing Director is the best person to blog.
Marketing directors can blog if their content would be relevant for their audience.
Yet some of the most compelling blogs I've read are written by the people whose role has nothing to do with communications or marketing. And who by writing can really capture the reader's imagination by describing their experiences or thoughts about a company or industry. The organic farmer blog on Stonyfield comes to mind. An organic farmer writes a blog about his experiences running his farm. The connection to Stonyfield Farms is that they make organic yogurt. What better way to illustrate the authenticity of their product by having one of Stonyfield’s farmers write a blog? In these days of advertising overload, such blogs clearly and simply illustrate the authenticity of a company's product.
If you were a marketing director and trying to decide if you should blog, there’s one yardstick to make the decision, would you be authentic if you blogged?