There was discussion elsewhere about the whole "write what you know" debate, and about how that relates to things like fantasy and SF.  Someone I know is taking a writing course, and the teacher appears to be discouraging the students from writing SF/fantasy on the grounds that it would be better to start with something closer to home.  (I'm paraphrasing a bit but that's the essence of the discussion.)

I and a few other people were of the feeling that if there's one thing that you're going to know well, it's a world you've created yourself.  Of course, I can see the devil's-advocate site of this argument: just because you've created a world doesn't mean it's automatically going to be infused with the kind of insight and observation that one gleans from real life -- and which makes any story all the more interesting and absorbing.  That doesn't mean you shouldn't be allowed to try, though.

More devil's advocacy: I've seen more than a few examples of people who dove into the deep end of the fantasy or SF pool quite early on, without quite knowing how they were going to swim.  In one of the round-table workshops I participated in, I had at least one young and enthusiastic writer who would present this whole massive family history and geopolitical analysis of their imagined far-off land, but wouldn't have a character for all of this to happen to.  That in turn requires the writer to have some idea of what people are like and why they do things.  You can see where this is going.

In short, I think I see why some insistence on sticking close to established reality, at least at first, wouldn't be a bad idea.  But if the writer wants to stick their neck that far out to begin with, why not let them try (and learn something from the experience)?  You could do lots worse.