Some people don't want their bid/asks to show publicly for various reasons. They could write a bot that polls mtgox to monitor the price and then post their bid/asks accordingly. This polling would be a strain on the mtgox website so mtgox implements dark pools to provide the same functionality in situ.

Whether dark pool users should trust mtgox is an interesting question. I imagine there are arbitrage opportunities for mtgox based on the fact that mtgox can re-order incoming currency transactions and make his own dark pool trades which he says are other people's dark pool trades. This would be hard to detect and very lucrative.

ByteCoin