I don't actually see any particular reason to use a singleton in this case: You are parsing a file, and want to store it someplace. There is no particular reason (from what you've said) why parsing a different file, or even reparsing the file should be prevented. (Besides the fact of memory useage/speed, which to me isn't a reason for a singleton, just a reason to keep using the same object.)
Basically, it's extra complexity to prevent future uses. I don't see why either would be something you want.