A WebmasterWorld thread has someone reporting that he noticed a robots.txt file has been indexed and marked in the supplemental index at Google.com
It is clear that Google indexes robots.txt files. The thread creator wants to know what is means if a robots.txt file is in the Google supplemental index.
I agree with g1smd that "There are no advantages or disadvantages. It just is."
So leave it as is and let Google worry about it.
Forum discussion at WebmasterWorld.