I have a NAS in my house and I still put a ton of important files on Dropbox. Before I had Dropbox I would suffer from pretty significant anxiety every time I went on a trip. What if someone breaks into my house? What if there's a fire? I would literally hide my NAS in my closet when I went away for more than a week. I don't treat Dropbox as a single source of backups but I do treat it as redundant storage to my NAS. It's simple and it just works without having to mess with a lot of settings.

Disclaimer: I work for Dropbox and I trust the people I work with with my files.