It depends what you mean by "smart contract", since this phrase has been used in so many ways. But if you mean "smart contract" as in general-purpose computation a blockchain, then an atomic exchange of native assets in MultiChain can replace one of the uses that people often cite for these smart contracts. But it's a specific mechanism for a specific purpose, rather than using a general-purpose computation engine, and this preserves the performance advantages of bitcoin-style blockchains like MultiChain:
http://www.multichain.com/blog/2015/11/smart-contracts-good-bad-lazy/
