Actually there are ways to run "smart contracts" without running every single computation on every single node like Ethereum does. Blockstack [1] (not affiliated, I just think it's cool) for instance runs most of the decentralized application off of the blockchain, but uses the blockchain for storage, identity, key management, etc.

That means you can have a decentralized application and also have the security of the bitcoin network, for example.

[1] https://blockstack.org/