If the dropbox client encrypted all files using a shared secret known only to myself, then my files would be protected by an additional layer of security where I manage the keys, that makes a lot of difference.
Obviously there is no such thing as absolute security, but that's not a reason not to take protective measures.
How would the dropbox client encrypt the files if only you knew the shared secret? You have to give the client your shared secret, and then we're back to where we began.
Obviously there is no such thing as absolute security, but that's not a reason not to take protective measures.