There's a good reason: I would argue that Dropbox has questionable security practices (by that I mean look around their site and try to find an in-depth description of how then insure individual account security... it leaves me with many questions :-). Dropbox stores and manages the encryption keys on their servers and they don't let users specify their own encryption keys. If someone manages to break into their serv…
Security is not goal in itself, although many IT types seem to think it is. As you note in your second paragraph, it's a sliding scale, to be weighted against productivity etc.