Jack has obviously been the glue that holds the Oceanic Survivors together on the island, and at first I felt a bit of annoyance sprout up when the show would focus an episode on him, but after the Looking Glass episode (last episode, season 3), I feel that it validated Jack in my mind.
I thought TTLG was Jack's best episode because he stopped talking and actually did something. This is my only favourite Jack episode though, all the rest have been annoying as hell.
Jack's Oxycontin addiction should not keep a young child from being turned off from wanting to become a Dr., nor should it keep people from enjoying Jack's role in the show.
You're looking too deep into the show now. Lost was never made for children, it was made for adults who could understand the storyline and understand what the characters are going through. I don't think the writers could give two crap-loads about a child that's watched Lost season 3 finale and now doesn't want to become a doctor.