Here is a thread that discusses how to combine targeted langing pages into a search engine optimization campaign. The consensus is that extra content pages are always good, even if they are relatively light sales pages.
Develop independent landing pages that can be SEO’ed and linked to from the main site itself. Optimized landing pages can be linked to from any page on your site, such as the sitemap.
Read the webmaster forum discussion at WMW.landing page optimization, landing pages, webproworld