If you are using an old sitemap but your site has many different pages and some of the old pages are deleted, what would happen? Would certain pages not get indexed? Does google punish your site?
The site probably does get a small punishment for links to non-existent pages. You should never delete pages anyhow. Always leave them up with at least a link to your homepage. If it's only a google-sitemap, and not meant for your users, the problem is not that big - but it does mean that you are not actually using that system to its best advantage.