This is not about a company closing down. Its about a company deciding to close their services for a user, arbitrarily, at their own discretion. Files you upload can be permanent deleted, at any time, at the whim of the service provider or any of their employees. You have no rights and no appeals.

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.