With no way to log off, that leaves a session hanging out there, which somone else could intercept.
As it is now, killing a session within your browser doesn't actually terminate the dropzone.com session. Because if you re-activate dropzone.com, you jump right back in to your previous active session.
The only way to actually kill the session is to kill the web browser, and then re-boot it to continue your work. This interferes with other work going on at the same time.