One of the main reasons:
Simplified Payment Verification (see the bitcoin whitepaper Section 8 ) becomes not usable. Since the miner will not verify whether a XCP transaction is valid like they do in bitcoin transactions, to check the validity of a XCP transaction, we have to track up to the very beginning (the address sent to burn address). This requires each client to download and keep the whole blockchain.
In Ethereum, they seems to find a way to solve this problem.
Detail can be found in their whitepaper:
http://www.ethereum.org/ethereum.htmlPersonally, I don't think this is something will make Mastercoin/XCP not usable. It just increases the downloading and parsing time.