I write in the Harry Potter Fandom, and since there are so few females as major characters, I write OC's into my stories that are fairly important to the story.
I have had people tell me they won't read the stories because of the OC's and I've had others say that the OC's were so well written, they became like cannon. It's a hard balance to strike when creating new characters in an existing world. Certainly you don't want to put a Mary Sue in, or give the reader less information on the OC than they have on the canon characters. I build most of my OC's around a central mannerism. For instance, one female bit her cheek when she was thinking, in trouble or nervous. This was a good indentifier for her and the rest evolved from that.
I think writing OC's helps stretch your wings as a writer. YOu can put a character in a known situation and have them react to it. It helps define what you want to express about the main character and can expand the sphere of the story beyond canon.
A badly written OC harms a story. If they are Mary Sues or Gary Lous it cheats the reader of a more complex reading experience. Perfection is boring so why write about it?