In practice, however, using BitCoin requires trusting something that is the equivalent of a gateway (an exchange) with your BitCoins if you want to be able to freely trade them.

The fact that this weakness does not appear in the BitCoin protocol, but is rather implied by the human desire for convenience and marketplaces, doesn't make it less of a flaw. One can sneer at everyone who lost money with MtGox for "choosing poorly", so to speak, but on the other hand exchanges are the driving force behind BitCoin's success.