Here is an excerpt from the discussion at the help forum,

JohnMu:

"If you are not submitting clean URLs in your Sitemap file, you'd be better off not using a Sitemap file. With session-IDs in there, it'll cause more problems (with us crawling and indexing those URLs) than if you just let us crawl your website normally (especially if you really have a clean URL structure). So my advice would be to either delete the Sitemap file, or make sure that the submitted URLs are really exactly the same, clean ones that we find while crawling."

Although this is not new to webmasters, it comes as a good reminder and for those who find it new should always remember it. Thanks to John for the reminder and also to Barry Schwartz from seroundtable.com who brought it out for more people to see.

The message is clear, send Google only clean URLs in your Sitemap file.