Sitemap is somewhat important and not a must if your website has a proper navigation for Googlebots already. However, sitemap could help your website being indexed quicker in some case, help some your pages to be indexed instead of non-index. It is not hurt to have a sitemap, both xml and html format in your website. I would give 7.5/10 for sitemap important.
Hahaha! I like your rating, Sx. I won't give it above a 5 because it's not really necessary for indexing and doesn't really have an SEO effect, as DoDoMe says: You're so right. Sitemaps have no effect on SEO, with the possible exception this indirect effect: if your navigation is otherwise poor, a sitemap can help Google or other SEs find all your pages. Sitemaps are very important for usability, though.
XML sitemaps have absolutely nothing to do with SEO from a ranking perspective. They are occasionally useful for indexing a site a bit faster but this only means that google has taken a look at it. XML sitemaps should submitted to check for crawl related issues and nail down any sort of navigation problems as someone above mentioned, but that is it. Nigel
Sitemaps are important. They tell search engines where pages are on your site, particularly useful if you use javascript for navigation and it is difficult for search engines to follow.
Sitemap is very important because if a person is not familiar with your website, or your websites have a lot of links that you can not show directly otherwise, he/she can go to sitemap and find exactly what he/she is looking for. For example: a corporation website have a lot of info and to get to the page that has specific info, going to sitemap can have a big advantage
A site-map is not only good for getting search engine spiders to follow your page links and easily crawl your entire site, it’s a useful tool for your visitors easily to the content that they want. There are many tools that can make it done autoly.
The simplest way to maximize your SEO strategy is to constantly work on adding keyword rich content to your website, and to produce “merit-based†backlinks. Search engines typically build their indexes by crawling links on websites. Search engines discover and catalogs web pages link-by-link. To get a site listed in the major search engines requires that a webmaster submit the home page URL to the search engines and then sit back while the search engines crawl the site to discover what pages exist. It’s the “sitting-back†part that most website owners struggle with. I’m surprised at how little awareness there is among webmasters about the importance of a sitemap. Visit, sitemaps.org, to learn the definition of a sitemap and the standard for creating and publishing sitemap XML files. A sitemap XML file is basically a listing of all pages on a website, including some basic metadata about each page. A properly-created sitemap can be submitted to a search engine that supports the sitemap.org protocol to help the engine accurately build its index. To my knowledge, Google was the first of the major engines (Google, Yahoo!, and Microsoft) to offer webmasters a way to submit a sitemap. Once Google has validated your sitemap, you are assured of accessing valuable information about your website including the keywords people click through to read content on your website, the number of links your site has (internal and external) and if there are any broken links, Google’s webmaster service will also tell you this. Of course, more information is available from Google’s webmaster service.
With Google Caffeine I wouldn't worry about telling Google how big your site is, or when it has new content. The web is evolving and it will pick it up even faster than before.
Sitemaps are worthless from an SEO perspective. That is not what they are for. They are used by the search engines to "assist" in crawling your site. They don't help rankings. Submitting a sitemap does not guarantee that a even a single page in your sitemap will be indexed. Sitemaps are totally unnecessary for most web sites. Having a good information architecture, good navigation, good interlinking structure on the site can accomplish the same thing a sitemap can. Sitemaps are good for 3 things: 1) Telling the search engines about a site that has zero inbound links and would otherwise not be discovered. 2) Telling the search engines about URLs that might not be found by crawling the site due to crawlibility issues. Some poorly designed sites have impediments that keep the crawlers from finding pages. 3) Prioritizing URLs on a site that has 100s of thousands or millions of URLs. In other words, telling the search engines which of those million pages YOU think are most important to be indexed if they are only going to index XXX pages. Sitemaps were designed primarily for #2 and #3 above.
Sitemaps help a site in crawling. By this Search Engine go to each inner pages in a manner for crawling.
Yes, sitemaps are very important. There are two types of sitemaps. The first type caters to search engines, the second caters to human visitors. With regard to human visitors, your sitemap acts as a table of contents. It provides your visitors with a list of links that offer quick access to various pages of your website. These sitemaps are generally created using HTML, PHP, or similar human readable browser languages. With regards to search engines, the benefits of using sitemaps include faster indexing and greater search engine visibility for your site pages. These sitemaps are generally created in XML format. When creating XML sitemaps for search engines, there is a specific syntax that must be used. Do a search on "xml sitemaps" for specific details.
I use both a yahoo and google sitemap for all my sites> I also add my sites to yahoo site explorer and verify with code. Make it EASY for the SE's to see you and your content, then the spiders will come more often and you will see positive results!