Are all mary-sues a bad thing?

I mean mary sues are needed in order to make a fiction novel a good one. For example, harry Potter is a marry sue and the story would of never been the same if he being an ordinary kid. Or Edward Cullen, nobody wants to read about Bella having sex with a ugly looking monster with 7 eyes, 3 penises, and 12 legs, no, readers want a handsome, strong, SPARKLING vampire. In my opinion readers prefer a mary sue, and authors create a mary sue because of two reasons: for the story to take shape, and because the pressure of readers of having a likable character.... am i wrong.....what are your opinions?