OK, I've been away, let's throw in my €0.02.
Backing up non-essential, downloaded files, should be completely unnecessary. Why? Because they're ALREADY backed up - on the servers of iTunes. Why should I make yet another copy of them? It's like printing out a copy of every mail you receive. Don't do it. It's a huge waste of my time, HD space, etc, to back them up. I buy the rights to a song, let me have my damn song.
Besides, backing up isn't always easy to do. I use my laptop almost exclusively now. There's one physical hard drive in there. I don't hook it up to 17 devices wherever I go - you really think I should devote 3 to 4 hours a week to backing up every little thing on there on a NAS or something? Good lord. I back up some 200 MB of personal files every week or so, and any larger things I'm working on very actively (usually papers, stories, thesis,... that I spend days and weeks on) every day and on several different places (mail them to myself, put a copy on a flash drive, burn one to a CD, whatever). My own stuff needs backing up, of course, but, just like no sensible person takes the time to back up stuff you can jsut reinstall (back up the save games, not the game content!), I don't see why I should back up stuff iTunes would give me, anyway.
(I only have MP3s that are copies of my physical CDs, but I'm a dinosaur on the issue of music.)