(This is a shorthand criticism, but since many HNers might not know it already: pretend you have a country where gambling is mostly illegal. Wagering on the roll of a die is illegal, but people really want to wager, so, you give them "investment" options like "Is the yen going to trade up against the dollar?" While you swear blind that this is investing and that the proverbial Mrs. Watanabe is making informed investment decisions after deeply analyzing the latest market trends, Mrs. Watanabe is actually just depositing yen with you and withdrawing less yen later. She doesn't actually own dollars at any point; she is just "trading" with you. You might not even own dollars at any point. The only purpose of the exchange rate is to be a source of legal random numbers, since a die or deck of cards would be a source of illegal random numbers.)
I haven't used DMM's FX offering but out of morbid curiosity I looked at a Bitcoin exchange which is widely reported to have copied it. It was like Zynga had made a pachinko game with slightly more numbers and less chesty mermaids. There was even a "bloop" sound effect when other people's trades went through.