Corporate blogs

What kind of companies should start blogs to communicate with their customers? My thoughts:

1. Smallish startups that don't have marketing budgets
2. Companies that have very few product lines, or even better just one product. That way the conversation can be 1 to N. N to N conversations are just noise!
3. Companies whose brands directly touch individual customers. I wouldn't recommend a steel maker to start a blog for instance. I would certainly like to see, say, a winemaker with a blog (like this one). People would like to talk to or listen to brands that they directly relate to.
4. Large organizations that have people who are mini-celebrities in their own right - Steve Jobs, Robert Scoble etc.

At the end of the day, a blog is just a medium. The message is always more important than the medium. So, unless you have something compelling to say to your customer don't start a blog!