The concern, which is fair, is that 1password's cloud is a target. And those targeting it have only one intention, which is to steal people's passwords and other information stored in the 1password cloud. In contrast, of course using the dropbox sync approach with 1password does put your information in the cloud as well. But, it's in your personal dropbox account. That dropbox account could absolutely be hacked, but very unlikely by someone with such clear intent to steal your 1password vault. Basically, 1password's cloud is the ultimate target, and your 1password vault in your personal dropbox account is not.

As a non-user of 1password, what's the value of the vault on it's own? don't you need a master password to unlock it?

Exactly.

A cloud you can control yourself.