For cold storage it makes sense, but I always consider UX - there's not enough solutions that make private key encryption, especially remote, as easy as opening a link or mounting to a directory.
I've used s3ql before, and it's really nice for making the encryption transparent. Not something pre-encrypting before dropbox upload can provide.
More, you wanna share those files via dropbox native tools? The recipient better have your private key or you need to reencrypt specifically for them.