If they are so timid as to not even rise to the challenge of applying for a job with some uncertainty, how can you expect them to deal with uncertainty in the field?
I would also point out that many good programmers don't like uncertainity (notably those with asperger), it is one of few jobs where it is a good thing. Not implying they are all rock stars, just that un certainty here is red herring.
Personally, I see any employer advertising for a position with stricter requirements than what they'll hire as advertising their own dishonesty. It just makes them look bad.
The fact that candidates that pre-select themselves out of being chosen is not really a big concern. Why should it be? The way around this is knowing exactly who it is you want for the job, but in that case I wouldn't be posting the job in a public forum. It's kind of what recruiters are intended for.