I think an advantage of using a WP install is that you can easily manage pages, galleries, etc. and you can also use plugins to handle forms. You won't have to

I think an advantage of using a WP install is that you can easily manage pages, galleries, etc. and you can also use plugins to handle forms. You won't have to start over when building your actual site if you build/use a WordPress theme. At my dayjob we build most of our client sites in WordPress so my starting framework is in WP (it's basically a combination of HTML5 Boilerplate + Blueprint + Starkers). When I need to build a non-WP site or convert a WP site to HTML/CSS, I go to my framework WordPress site, view the source and save it as index.html. There's only a few things that need to be changed, like the path to the stylesheet and images.