Personally, I like Mary Sues. They can serve as wish fulfillment, but somewhere around grade 11 I began to use Mary Sues as imaginary test dummies to act out scenarios etc. I started adding more conflict and realistic challenges for them to face, and then added people I've known or had conflicts with. Not only did I learn how to write interesting plot driven stories, but I feel like my intuition has developed as well.
Everyone should feel free to create a world where there is absolutely no limits unless they set them themselves.