I should look more into setting up NAS system... maybe when I get my own place; I'm not sure that I'd get much utility out of it over manually backing up with a physical USB connection at this point.
It's more for additional redundancy and availability. The NAS itself won't solve the problem completely but adds another layer of availability w/ redundancy (e.g. you can have four disks [or more or less] simultaneously synced up so if one goes down there are still drives that have the data). The purpose is generally to increase 'fault tolerance' and lower complete failure rates. For most people and situations, it's probably a bit overkill but it's mostly about the risk you're willing to tolerate. Buying extra space for backup (and of course using it) is similar to buying insurance in a sense... of course, you could just get a second or third USB drive and figure out a rotation for backing them up (and as someone mentioned, leaving a copy at work and one at home, etc) but it won't be as seamless as with a NAS. The concept and methodology behind backing up to a NAS is a bit different than backing up to 3-4 individual hard drives one-by-one, but they all seek to accomplish the same goal at the end of the day.
If you spend money on the right NAS though (e.g. Synology) you can get some pretty decent quality from it as a streaming media server for movies, recorded tv, etc... the NAS I have by Netgear isn't really intended for that as much as it is just for file storage and backup. But even accessing files on it on a daily basis (from Mac OS) stinks. I think it's a little better in Windows though. As far as data backup is concerned though, it does a good job.
I would warn: Consumer grade RAIDs are really not all they're advertised to be. You're just as likely to get multiple failures, especially on very large RAIDs. When one disk fails and is replace, the rebuild often walks through long unused sectors and triggers multiple failures. In the consumer grade market, I think RAID generally adds complexity and price with very very little payback in actual recoverability.
Also: remember that RAID is not a backup. It's redundancy. If you delete a file, it's gone.
A real backup plan will give you multiple versions going back, so that one "oops" doesn't torch your entire data set. It's not that uncommon to have a file where you need 3 or 4 versions back.
