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The determining factor for inclusion of links into an HTML sitemap is not the depth of the link. For example this Q&A thread is 2 levels deep (http://www.seomoz.org/q/what-are-the-benefits-of-footer-expanded-site-maps) but in my opinion should not be included in a sitemap. The primary purpose of an HTML sitemap is to help users navigate your site. It is an alternative method of presenting your navigation bar along with other important links within your site. A secondary purpose would be to help search engines navigate your site (although the XML sitemap is better suited for that purpose). You can view your link statistics through an Analytics tracker. Google WMT will show how often users click on the sitemap. With respect to it being a good practice, I would say it's not a bad practice but also not a required practice. I place it on all sites I work with because many clients ask for it. If your site offers solid navigation, there is no need for a sitemap at all (XML or HTML).