hi there,
arash from dropbox here. all data is (as we state in the referenced help article) encrypted before it's stored on the backend. I'm not sure why you're concluding that de-duplication implies lack of encryption. the de-duplication occurs prior to encryption.
all data on dropbox can be made shareable and is web viewable. as a consequence, we do need the ability to decrypt in the cloud.
re. employee access to files - there are controls to prevent this. for example, even drew (founder/CEO), doesn't have physical access to our storage servers anymore.
for very sensitive data, there's always the option to use truecrypt (we even offer this as a recommendation in our security documentation: https://www.dropbox.com/terms#security)
all data on dropbox can be made shareable and is web viewable. as a consequence, we do need the ability to decrypt in the cloud.
re. employee access to files - there are controls to prevent this. for example, even drew (founder/CEO), doesn't have physical access to our storage servers anymore.
for very sensitive data, there's always the option to use truecrypt (we even offer this as a recommendation in our security documentation: https://www.dropbox.com/terms#security)