Very important. Accessibility, usability, portability, it's the law (in some instances), scalability, easier to integrate with various application frameworks and CMSes. The list goes on but other posters have covered the points well enough.
Your average business owner typically says "who cares" much like the OP. My most effective response thus far has been "the Google bot is nothing more than a blind end user. Don't you want it to list your site effectively?"
