The counter-argument would be that they would expect a panic sell (or at least recognize it if it happened) and so should be willing to wait a bit to sell as the market normalizes. So if Pirate does repay people, you would expect a significant medium-term downward pressure on the price of Bitcoins as more and more investors pick that time to cash out.
While I would agree with you regarding most Bitcoiners, the kinds of people that invested in Pirate seem to be anything but rational. Have you seen their posts and how quickly so many of them began auctioning off 1000k BTC accounts?
I think he's pretty much gotta pay in small increments and get to the larger holders last in order to keep the price from completely tanking. It's funny, he appears to actually be buying back the loaned coin as his own customers sell them back to him, so the more he returns, the cheaper he gets it. If Pirate isn't the guy hedging the price for Silk Road vendors I'd be extremely surprised at this point. It seems pretty blatantly obvious now. I think for the last year we've clearly been underestimating just how much money is going through Silk Road.