I have been searching the forums for a solid, positive sign, brought from implementing Google Sitemaps for a site that is already fully indexed. Member Relevancy at Search Engine Watch Forums posted a response stating;
Something else that appears to be cool with Google sitemaps is tht it appears to take pages out of the supplemental results. I just recently added a sitemap for one of my sites that was fully in the supp index and a few days later half the pages got taken out.
Now, sites within the supplemental indexed are arguably not fully indexed. But I know tons of developers have issues getting these sites out of that index. If it is as simple as feeding Google a Sitemaps file, then it might be worth the effort.