The difference is that a user doesn't feel like just using his computer normally gives him any backup. But if his folder is inside Dropbox, he might be lulled into a sense of comfort since he thinks of Dropbox as a kind of backup[1]. So if he uses Dropbox instead of just using his plain old computer (hard drive), then he might start to get more careless than when he didn't feel that he had a safety net.

[1] Though personally I would disagree.