That'd work unless you:
1) have a ton of movies or mp3s, in my many of which were ripped from Vinyl that you can't buy in digital form
2) have a ton of pictures or home vids of the kids and such that you couldn't reacquire in any event.
3) have a ton of other stuff, things like Installations, Dev stuff (I'm a programmer), and whatnot that would be difficult to "reaquire"
4) have a spent a bunch of time cleaning up image files, rom sets, etc. Sure, I could probably re-acquire all the files later, but when you're recovering from a nasty disaster like a downed machine, the last thing I want to do is ALSO redo a ton of work I did cleaning everything up in the first place.
USB drives are so cheap, it will often make more sense to pick up 2, backup regularly and just rotate them offsite (say to my parent's house, or a safe deposit box) on a monthly basis or so.
Even as an IT pro, systems can go boom to the point where they aren't recoverable. I got lucky with the lightning strike recently in that, in my NAS, which had two mirrored drives, one drive survived intact and I could recover it. And it had the lions share of my recent changed data on it. A backup disk had all the older stuff.