I think for many it isn't an issue of having not made at least one back up of purchases, the problem is, new OS, reinstall, equipment failure, reinstall, heaven forbid some kind of home disaster and if lucky eventually, reinstall, need to format, reinstall... back up to external drives, they fail, burn to discs, they have a limited life span, damage to your home, back up is gone.

That extra security of a downloadable account history can be pretty important when a lot of money is spent on either content or software.