A Ponzi or MLM use a single source of supply to create a profit center. You can buy bitcoins on the open market. That would be like buying your competitors products to sell and would kill your pyramid.
The Ponzicoin process is not for buying bitcoins. It is only for spending them upline, with the idea that in the future someone else would sent some to you. All the standard methods for buying and selling bitcoins would be built right into the client, since the client would only be a slight modification of the original Bitcoin-Qt client.
