For now I am removing all my Sitemaps from Google. From my experience they only work to keep pages out of Google's index. See, my problem is that almost all the pages on all of my sites are dynamically generated from content provided by a database, API's, feeds, and the like. So, keeping an updated Sitemap on a dynamically generated site with 10's of thousands of pages becomes a burden, especially when the site content is provided via API and you just have no idea how many pages are on your site and what they all are. There is just no good way to keep an updated Google Sitemap for sites like this - even if automated.

 I have tried third party sitemap generators that you run and they crawl your site like a search bot, building the XML the whole way. As many pages as one site can have, it can take many many hours to build the XML to the point that it contains all of the pages. Then, a day later, there are 1000 more pages, so it needs built again. And here is the important part. If you submit a Google Sitemap and you are missing pages from your site in the file, those pages missing from the XML won't get indexed. I don't care what Google or anyone says, this is based on raw experience, and Google will only index pages that are in the Sitemap if you submit one. You can have everthing else wide open as far as no follow, robots.txt etc and they won't get indexed if they aren't in the XML.

Case in point: I recently waxed one of my Sitemaps for a dynamically generated site from my Webmaster Tools account. I hadn't updated the Sitemap in a while and so pages weren't getting indexed. Two days after deleteing the sitemap, all of the pages where indexed. I've seen this over and over again.

 From my personal experience and perspective, the only good thing about using a Sitemap when it comes to Google is using it to clear up problems you see with Googlebot crawling your site. That's it. If you don't have problems with Googlebot crawling your site, you are better off just not using a Sitemap, and let the bot run wild. Use robots.txt to block whatever needs blocked.

Google, SEO