I think the Mary Sue syndrome is a normal stage that all writers fall into when they are beginners. You write from the heart, essentially your daydreams, because they are the clearest visions you have in the beginning.
They become a real problem, however, when ALL of your stories are Mary Sue tales and you never outgrow that point of view. And frankly, (I think we can all agree) most of these stories are BORING, because we really want to read about the stock LFN characters. (At least I do!) Unfortunately, a lot of writers don't even recognize that they are writing Mary Sues, and just keep doing them.
The closest "LFN" Mary Sue story I ever wrote was Savannah, and it ended up being close to a Mary Sue because my co-author wanted it that way. Personally, I don't like matching Michael up with strangers. I'm a HR who wants Michael with Nikita--period, amen. (I don't offer to co-write anything anymore for that reason.)
Whenever I run across an obvious Mary Sue, I usually stop reading and go on to something else. They are fine if you are only writing to entertain yourself, but most of us want to write for the enjoyment of others once we get more confident of our skills as a writer.
