I'm not convinced by the sitemap rhetoric at all. Like I've posted elsewhere on this site, I can see the Googlebot checking a sitemap, then fetching some URLs that are obviously acquired from previously retrieved pages, not the ones in the sitemap. I'm wondering if it's some interaction between the lastmods in the sitemap and the If-Modified-Since behaviour of dynamically generated pages?

It seems like they only help Google - maybe - , not change the way it crawls your site. There's a bit more info here:

What is a Sitemap file and why should I have one?