If you can afford it, and you have RAID, why not?
The problem with RAID 1 though is, even though it basically mirrors one drive to another, you still only have 1 physical drive to Windows, so for example, if you got 2x 300GB HD, Windows will see one drive with 300GB.
Another problem is, if one drive got a virus and you started losing data, the other drive would follow suit.
On a good note though, it does save you from losing data if the misfortunate event of one dying happened, which I've had happen alot.
Other RAID options include RAID 0, which basically takes 2 drives, and then shares information between them, speeding things up a bit, but if one goes, you've pretty much lost your data all over again, with 2x chance of something going wrong.
RAID 3 I think, mirrors and stripes, but you need a minimum of 4 drives for this to work.
Abit IP35 Pro / Q6600 G0 / Zalman 9700 / 8800GTS 640mb / 4x 2GB Corsair XMS / X-FI Xtreme M / 1x 1TB / Antec 900 / Logitech Z-5500 / Samsung 20inch