Well, since you are running SOFTWARE raid, your PC need to get to the OS and know about your HDD configuration before they can boot. Since mirroring is just copying one drive to the other, the HDD configuration is the same and so you can still boot off it. But for stripping, your HDD's are stripped into one big HDD, and if you cannot access the OS to let your PC know that, your PC will think your HDD is all messed up. That's the simple explaination of why you need your OS in a unaffected drive so that your PC can talk to the OS before recognizing the raid configuration.
As for CPU utilization, actually I think Tomshardware did a benchmark on that, and software raid actually have pretty much the same utilization as the IDE raids. So if you say software raid is a waste of time, IDE raid is pretty much a waste of time too. (A lot of people are thinking that way, but that's another big debate) As for my self, I had my OS in my SCSI drive, and strip my two IDE drives via software raid, and give myself free performance gain. But right now, I am not doing that anymore, since I need to dual boot win98 and win98 couldn't recognize my stripped partition. Darn!!