I have said this in another thread, but it bears repeating here...

What if pirate sent 500, 000 BTC to an invalid address? It's got to be the most extreme volatility you could subject the system to with that amount of coins. I do get that the 20.5 million BTC left will be worth more eventually, but the immediate aftermath of such an event would be more violent than any pirate antics so far (and easily exploited if he only destroyed some coins, as it would still give him BTC volume to make the market dance with).