If you didn't back up your CDs and they got stolen, you don't get a replacement disk from Best Buy.
If you don't back up materials which you buy online, you are treading on very thin ice. I backup everything I create, and I back up everything I purchase. The latter takes much less effort and backup media than the former.
eMusic will allow you to re-download songs you've purchased online; they are one of the very few with this policy. It's great. Still, if you're relying on that ability instead of backing it up yourself, what happens when eMusic changes its business model or gets bought or just plain fails? Do you really want the security of your purchased music tied forever to the solvency of a particular business (the same argument which is made against DRMed music purchases, although I'm a bit more confident in Apple's continued solvency than in eMusic's!)
In any case, it costs close to nothing to back up your data. You should be backing up your data anyway, or you have much more major concerns than if you'll be able to listen to last year's Kelly Clarkson single after your drive crashes. Adding purchased music to any reasonably organized backup system should cost you no more than 15 minutes. Apple's own Backup even has a preconfigured option just for this, which takes about thirty seconds to open, activate, and verify that it's set up correctly.
All that having been said, Apple has a non-public policy of allowing redownloads of purchased music. However, moreso than eMusic's public policy, this could change at any time, and is quite specifically not a part of the agreement you make when buying music from them. Don't rely on it. But, Apple's bent over backwards to help people out after a crash in the past.