The Sitemaps.org deal (I'm assuming this is what you're talking about) is basically for sites that are having trouble getting all of their pages indexed. If you don't have a problem with that (assuming not since you were getting top rankings), then you really shouldn't have a reason to use it.
It's also possible that there was some sort of a syntactical error in the file, making it drop portions of your site from the index. You always want to make sure you get the file to validate using the available tools. Errors in that file can indeed cause problems.
Brian.
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