Yeah that helps if your machine or something in between is not compromised and the other side is not too interested. If this is not the case, your off-line data is still off-line and not reachable while data in the cloud is easy obtainable by the attacker so he can do with it whatever he wants.
Afaik at least Crashplan uses strong asymetric encryption so you can store your decryption key offline. If a party is really interested in your data they might as well obtain it with a wrench. Don't get me wrong, I do think you're right about this. It's a tradeoff between convenience and security. Or viewed from a different angle: Is it worse that your data might be read by a third party or that you lose your data because you forgot your manual backup. And which scenario is more likely?
