Keeping data is crucial, there's no doubt about this. Data backing up has evolved from as painful as copying files onto a floppy disk to an eye candy with Apple's recent invention of the Time Machine.
However, on the other hand, completely losing data is equally important, when you decide to let go your old hard drive. Trashing files from within the operating system generally doesn't make the information completely go way. And you don't want it to be retrieved by people with ill intention.