I can cite four examples of where this is not the case for me:

a) iTunes purchased music & video. Once you download these, you (generally) can't do it again. For a few years, we bought a lot of media from Apple via their iTunes store, with music and video it's about 60GB of data I can't replace with a re-download.

b) Family Videos. I have about 200GB of family videos, dating back to the 1980's as transferred from old VHS tape, Mini-DV and more recently SD cards in various digital cameras. I can't get those back, especially the old VHS tapes which have degraded to the point where they don't play.

c) Scans of bank statements, health expenses, utility bills and the like going back to 2000. That's about 10 GB right there - I scan to 300 DPI TIFF, usually, for best print reproduction and ease of conversion to other formats.

d) Family Photos. We shoot at 3 megapixels, normally, and occasionally at 6 Megapixels. Our photo library, which includes originals and modified versions of images, is currently 64GB.

All told, my media library, plus misc documents, work related stuff, and so on, weighs in at around 500 Gigabytes. Fortunately, USB disks are relatively low cost so that's not an issue. Every computer gets it's own USB drive of about the same capacity of the internal drive.

I'm a big fan of automated external disk storage, as we tend to use laptops a lot. On the Mac side, we use SuperDuper, which makes a bootable image of the entire disk, this is by far the way we like to make things work. We do have a couple of PC's, these back up only the "My Documents" directory to external USB Drives, as these are single-user machines, that's all I care about.
Two systems run Ubuntu 8.04, and for that, I have sbackup running, which is more than good enough.

EVERYTHING also backs up to a 500GB disk (it's actually 2x 500GB disks, set up as RAID1) set up as Network Attached Storage, that disk array is actually located in a small refrigerator out in the shed, along with a set of DVD's with a backup of applications. That's the "house burns down" storage center. Very old media, stuff that isn't likely to change, is also burned off to DVD where possible, many of the files are too large for optical media (especially home videos), so they live on Magnetic Media.

It's a lot of work running a data center at home.