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If your site has properly designed navigation there is absolutely no need for a sitemap.
If you are at all concerned about accessibility, sitemaps are important. Many styles of dynamic navigation are not accessibility friendly, e.g. easily read by text readers for the visually impaired. Another consideration is that it provides a backup in the event your regular dynamic navigation fails. Remember, not everything works equally well in every browser, or for that matter device. Besides, it cannot hurt.
I see sitemaps as the easiest way to know roughly how many URLs you've got indexed, thanks to the Webmaster Tools report.
now that Google has so many more sources of URI discovery, it seems like the diagnostic value of not having a sitemap is lost, but I always thought it an interesting counterpoint
Sitemaps are a way to tell Google about pages on your site we might not otherwise discover.
Sitemaps are particularly helpful if:
* Your site has dynamic content.
* Your site has pages that aren't easily discovered by Googlebot during the crawl process—for example, pages featuring rich AJAX or images.
* Your site is new and has few links to it. (Googlebot crawls the web by following links from one page to another, so if your site isn't well linked, it may be hard for us to discover it.)
* Your site has a large archive of content pages that are not well linked to each other, or are not linked at all.