It is a drawback, it introduces additional layers of LIQUIDITY that simply won't be there
You need base currency liquidity (just assume bitcoin), you need mastercoin liquidity, then you'll need your Masterbitcoin liquidity, then you need liquidity in the ASICMiner stock
god forbid you were trying to trade options on that stock!
Not sure if we can accurately assess at this point the demand for such services. How strong will demand be for a decentralized stock exchange? Will it be strong enough to gather enough liquidity? I don't see why not. People don't like very much the current offerings and they might flock en masse to mastercoin exchanges.