Dropbox Inc. seems to think that I want its client to start automatically, always! Even if I manually remove the dropbox entry from startup applications or turn it off, it will be "magically" re-added/-enabled simply by running the dropbox client once. Well, I still prefer to stay in control of my computers (rather than the other way around) and thus keep dropbox out of autostart with a stupid "sudo chown root:root ~/.config/autostart/dropbox.desktop". Done!
Btw. this considerably helps reduce battery drain while being mobile.