Well, I don't have anything positive to say about submition of sitemaps...
I had pages from two different sites dropping out of the index (at the same time I noticed a huge decrease in traffic), that was the situation after submitting sitemaps in Webmaster Tools.
The one of my two sites where (at the time) just about 6 months old and had a good, increasing traffic, until I decided to submit a sitemap in Webmaster Tools. After submitting the sitemap, pages began to drop out of the index and traffic decreased by 50-60 percent.
Within a week after removing the sitemap from Webmaster Tools, traffic slowly came back. Within 2-3 weeks the dropped out pages began to show up in the index again. I didn't made any radical changes to the website at that time, so I'm pretty sure that it was the sitemap submition that had caused the problems.
Today, both sites are doing very well -- without having any sitemaps submitted, of course ;)
[edited by: OutdoorMan at 1:22 am (utc) on April 27, 2008]