If you juggle around a lot with big files, trust me, a raid stripe can be invaluable. Try compressing/decompressing a big file from one harddrive to the same harddrive - then try the same with two separate harddrive, or a raid stripe.
Fortunately single-drive performance has been getting a lot better the last few years, but if you need high sustained write rates, it's hard to beat stripes. And while they don't reduce seek time in my experience, they do help against disk thrashing when doing multiple things on the same disk.
Mirroring (and raid-5 and raid-6 etc) is not a substitute for backups, and only retards use it that way. They're intended to protect against disk crashes before the nightly backup, so you don't lose a day's worth of work. And they're intended to be able to keep your data available while hotplugging a spare disk and replicating your array, as opposed to shutting down the server, replacing disks, and restoring from (last night's) backups.
I've got a 10k rpm raptor drive as my system disk, where I keep windows and my apps, et cetera. Low seek time = love. And the disk has very good sustained read and write speeds as well. Then I have two 160gig so-so performing maxtor disks, which are supposed to be in mirror for all my data (which I don't back up near often enough >_<), but currently they're forming a (filled >_<) 320gig stripe, because I wanted to play around a bit.
Btw, software raid isn't all that bad - for consumer on-board raid, lots of stuff is being done in software by drivers, and performance is still decent. It's only if you want big and really fast arrays that hardware solutions are really necessary. And if you want big arrays, you're probably doing it for a file server, that probably won't be spending CPU cycles on much else... so the hardware solution is really only if you build a weak system, or if you go all the way with battery-backed ram on the controller for safe caching