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First, do you really NEED a site map? Some sites do, others don't.
If you are using dynamic links of any kind, or some other dynamic method of page linking, you absolutely need a site map so the search engines can find your pages.
If your pages need specific filters for indexing, a site map is probably a good idea, too. Setting the correct META tags for each individual page can be a pain alleviated by using a site map instead.
Otherwise, I don't see much value in a separate site map, especially if your site uses static links (either relative or hard) and is WELL LINKED internally.
My opinion is based primarily on my experiences with my website. I have something over 4000 pages with thousands of links and cross links (most relative, rest hard) throughout the site. Google has never had a problem finding and indexing every indexable page on my site. The only downside that I have found is that other search engines often ignore the ROBOTS META tag on the 100 or so pages that should never be indexed. I don't know if a site map would solve that problem or not.
mmerlinn
"We've found by experience that people who are careless and sloppy writers are usually also careless and sloppy at thinking and coding. Answering questions for careless and sloppy thinkers is not rewarding." - Eric Steven Raymond