Part of SEO today is generating linkbacks. To do that, you need an easy-to-bookmark, easy-to-remember, easy-to-copy : aka: an easy to link to url. Most designers and optimizers will tell you that your changes of getting those links backs goes up by multiple percentages when you use a standard file extension. It has been my experience that people will not link to nonstandard extensions nearly as often as htm or html extensions.
As far as Google is concerened it does not matter. I don't have extensions at all. All my URL's are www.domain.com/?pane2=blah_blah. Google spiders my site all the time and I have all my pages listed on Google. I don't know about what Brett says but it makes sense if you want people to link to your internal pages.