Dropbox is a decidedly single-user solution. Try running a corporate network of 20 people using add-hoc cloud-based file-sharing with no file locking. And try doing it without getting "Why can't I access xyz file?" or "What happened to my data?" questions every 15 minutes.
File locking sounds like technical minutae until you realise it's probably the single biggest feature requirement of a reliable storage solution. Use a commodity NAS instead of you want free/cheap (just be wary that your backup position is likely to be unknown/untested, but that's probably on a par with giving the task to someone who doesn't know WTF they're doing on raw linux or M$ platforms).
BTW there are some annoying limits with GMail for domains. Again though, beats trying to be a mail admin who doesn't know wtf they are doing :P