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How critical Google Site map is depends on how dynamic your site is and how deep Google is able to index that site.
If your site is quit large, made with a CMS (like most large sites) and the way the links are made in this CMS, makes it hard for Google to index your site, a Google Site Map can remedy this problem.
If, how ever your site is a static html site and Google has indexed all of the content, I donít think a Google Sitemap will make much of a difference. It will also not hurt your site.