(Digging back into my gray matter for when I was a server admin as well as a SAN storage admin)
Another benefit of raid is the increased number of read/write heads. We used to take servers that needed really fast disk access (database servers for example) and lets say that they needed 1 TB of data. we wouldn't throw 12 100GB drives at it (never format to 100 GB and you lose 1 for the RAID 5). We would put in a big array of like 24 50GB or in one case, we had a monster with an external set of arrays and several (not just dual) SCSI cards to communicate to 35 30 GB drives. That system had multiple on-line hot spares since the more drives, the more potential points of failure, but boy was it fast! SOOOO many read/write heads.
Of course, you have to have the CPU processing power to handle all of that data too.
So either you can do some sort of RAID setup and while you don't have a backup (be sure not to delete or format things that you don't mean to), you should gain zero down time due to a hard drive failure, and you gain increased data throughput by having the data striped across multiple drives.
Not saying that it is the best way, or only way, just offering up other benefits of RAID. Most people I've talked to that have a HTPC just put a couple of large drives in the HTPC and then backup the key data elsewhere (another computer). In essence it is like a remote-RAID system in that you are still spending the money on storage backup data on another system if you keep things stored in more that one location. If you just have 1 copy of the data, then you run the risk of losing it... So on and so forth.
