Why Back Up?
Because They Don't Care About You
Technology is business, and in business, money often takes priority over ethics. With the law perpetually lagging behind technology and business, there is often nothing between you and getting screwed over besides your own volition.
There is real convenience to web services like Google Apps, but convenience should be evenly matched with user control and agency. Businesses can be extremely helpful, but they are also self-interested. As benevolent as Internet-related often present themselves to be, your data is valuable to them. And it's valuable to you, too.
Luckily, a few basic (and cheap) precautions can bring the long-term care of your data into your own hands, away from the short-term world of the Internet.