There is a WebmasterWorld thread named Removing the Google sitemap got all my pages indexed. People in that thread tend to be piggy backing off each-other, saying that after removing their Google sitemaps file from the Google Sitemaps product, their indexing issues (i.e. pages being in the supplemental index) have improved or more stabilized.
Should Google Sitemaps be blamed or attributed to the improvement on ones indexing in Google?
Reviewing these cases in the threads, without specific evidence or examples, I strongly feel Google Sitemaps is not the issue here. Many other sites have recently improved in terms of not displaying the "supplemental result" near the search listing during the time frame of this thread.
I believe this is just one of those common cases of timing and coincidence.
Forum discussion at WebmasterWorld.