Mary Sue isn't an easily defined term from what I found and discovered over the years. It can be well written. I have read stories where, by these tests, the characters are Sues but the writing was good. The characterisation was good. Things fell through like liquid instead of being shoved down your throat like an eating contest. Case in point, I'll use one of my own as an example. It was when I was getting the hang of writing (slightly translated into training wheels from a tricycle), so I started towards the Anita Blake section. This was loooooong before LKH went crazy and made me wonder "huh?" at the beginning, middle, and end of her books. I wrote in an OC. I wrote in plenty of OCs. I also crossed the thing over with Merry Gentry because I had a mystery that I wanted to explore that A Kiss of Shadows didn't. Looking back, it had a very mediocre start. Very much unimaginative by the state of name and characteristics when you compared it to a number of stories in the same category. And do you know what happened? The story snowballed. Reviews piled in as well as emails (especially my favourites of 3-paged emails). I remember sitting in shock, finally coming to realisation that a teacher had told me ages ago before then, I could write. Between chapters 1 to 35 (my first long story), my beta could tell me how she knew I was improving in writing. Putting chapters 1-5 next to chapters 17-21 was a vast improvement in style, characterisation, grammar, spelling, tone, and more. Heck my vocab had gotten larger from looking up new words.

As for criticism, some people don't take them too well. Others love it for the improvements they can make. Like fiches, you have to sift to find the ones that will help you. Relaying back another experience, there was story where the main character was the Sue that many fear and reject. Trying to be helpful, I told her that and tried to suggest things of why she might be getting 0 reviews. Such as not having the world bow down to your character's feet. She took the story down the next day and I felt like a heel for doing that. I was trying to help and it blew up in my face. There are people out there who would enjoy ripping a Sue to pieces. That isn't going to help. It's going to make the writer spiteful. It's going to make them clam up some more. They might be a very good writer buried deep down but because they committed the Sue sin, they might never try to surface.

Getting to the point, an OC, Mary Sue or not, is like fanfiction. It's a learning tool. It can make and break you on its own without people trying to smash you into the ground because you are not following their rules. Also, by inserting in a new character, it can help you develop a thicker hide to criticism and especially to flames because in the publishing world, you'll find reviewers who will tear apart your story soon after it's been advertised and sold on Amazon if you get there. Just look at a couple of the reviews given to White Night by some "professionals" who don't seem to have read the book.