Dropbox doesn't encrypt your data. Compare them [1] and SpiderOak [2] for example.
https://www.theregister.co.uk/2011/05/16/dropbox_ftc_not_goo...
Since you’re running their software, you’re ultimately trusting their policies anyway. Even if they had encryption at rest it wouldn’t tell you that it’s correctly implemented and doesn’t e.g. leak keys.