A lot of password managers seems to promote client-side encryption as a key feature of their service. For example : LastPass, Firefox Sync, PasswordBox, etc...

They all say something like

> 
> Even we don't know your master password and cannot decrypt your data,
>  hence it is more secure.
> 
> 
> 

Making the user think that it protects him against the password manager he is using.

But, in reality, they control the code that does the encryption on the client side and can change it whenever they want to instead do the encryption on the server and you would not notice anything.

So, does it add any security?