What happens if you apply to YC and another accelerator and need to respond to the other accelerator while you are waiting to hear back from YC?
People who make exploding offers are not just bad, they're also acutely aware of that fact, thus the manufactured scarcity.
Not that I agree with that interpretation; I think YC's viewpoint makes a lot of sense -- especially if it is a proven negative indicator of success.
[1] "If you apply to other accelerators, then we'll be more critical of you."
This is similar to the notion that a startup can "sell the dream" up until the point where they have revenue, after which they're no longer selling dreams, they're selling the trend.
Make a decision, and then make the most of that decision. Sam is not saying it's bad to do other accelerators. However, for very natural reasons, graduates of other accelerators have more expected of them. Likewise, I doubt YC would accept their own graduates for another round.