Seth Resnick relates in Where's My #*@@ Pictures an experience he had with a 'cloud' backup. The company decided to throw in the towel, and he had to scramble to retrieve his data. Could very well have lost much, if not all, if he hadn't gone to great effort and expense.
Myself, I'd never rely on a web solution, especially in this day and age of corporate fickleness. Companies vanish in the wink of an eye, or get absorbed by a bigger fish who may not have the best interests of the established customer base in their plans.
You are also trusting the security of your data to a third party, with virtually no control, who's primary concern is the bottom line.