I'm not saying bitcoin protocol needs to be changed in order to support protocols on top of it. Is it possible to do what mastercoin try to solve without introducing another coin, by using the same approach of embedding information on the blockchain ?
A special currency is not required for "smart property" decentralized exchange. We implemented it a year ago for colored coins, but masrtercoin-like implementation is also possible, and doesn't require Mastercoin.
However, other features require a special currency:
1. Escrow-backed currencies: you need to put something into "escrow", and it cannot be Bitcoin, because Bitcoin doesn't obey to "escrow" rules.
2. Automatic bet and CFD settlements: again, you need special rules, and thus special currency.
3. Saving addresses, spending limits.
However, a special currency is necessary only if you want to implement it exactly the way Mastercoin does. If you consider different kind of implementation, it is possible to do it with Bitcoin alone:
1. CFD/bets: Can be done via Bitcoin contracts (
https://en.bitcoin.it/wiki/Contracts )
2. Savings/rate limiting: can be done using multi-sig scripts and third-party operator which will control one of keys. See here:
https://www.bitgo.com/It is very hard to implement escrow-backed currencies in a decentralized way without a special currency... But if you consider complex approaches, it's probably possible to do this using secure multi-party computations, you can create a "decentralized autonomous corporation" to take care of escrow. But to do this, you'd have to hire highly qualified cryptographers to design the system, it's not so simple.
That said, escrow-backed currencies are a bad idea in the first place. (Don't take my word for it, Ron and David wrote that CFD's are more likely to work properly.)
Anyway, real motivation for Mastercoin is this: "Personally, I am waiting for an investment opportunity which offers the kind of blue-sky moonshot that bitcoin is, where my investment gains value in proportion to the success of the protocol. " -- J. R. Willett.
See here:
https://groups.google.com/d/msg/bitcoinX/9_YJ_3qi41c/PWEp1WI4E10JWho cares if there is another way to do this if that other way won't make you rich?