Except this Bitfinex thing shows that while an amount of bitcoin can't be manipulated or counterfeited, their actual value is relatively easy to influence because it's mostly unregulated. I mean think about it, if Bitfinex is lying about their dollar reserves this might be one of the simplest and most efficient scams of all time. They're literally making up money as they see fit. Hundreds of millions at a time. Just like that.
Now of course maybe they're not actually crooks and they do have the money, it's kind of a schrodinger's cat scenario at the moment. Still, the fact that nobody manages to know for sure the truth of the matter shows that they could be doing it and we'd be none the wiser. People are buying and selling for million of dollars of these things, not knowing if they're trading digital dollar bills or monopoly money.