MasterCoin will not allow the trading of assets against BTC on their DEX - assets could only be traedd against MSC. On his blog David Johnston (Chairman of the MSC Foundation Board) presents this limitation as an advantage, and later in the comments he claims Counterparty has the same issue - requiring XBTC for trading on the DEX...

http://en.reddit.com/r/mastercoin/comments/2g4mmp/utility_is_the_mother_of_liquidity/