The MAIN aspect of offering a RAID system is for redundancy. It's the FIRST letter in the name. Different RAID modes have been able to improve the performance (just because of the number of read heads available). They've also been able to provide higher data availability because of the redundancy of the system, but the initial goal was to provide redundancy. People at home DO also need this. I have a NAS on my home network that acts as a centralized file store. I've had it this way for MANY years. All of my photos, music and videos are stored there. My NAS has 2 drives in it setup in RAID 1. That saved my butt. A few years ago, one of the drives in the RAID died. If I had not had that second REDUNDANT drive, all of that data would have been lost. My brother just went through this exact same problem, but his wasn't a RAID setup. Guess what? All that data is gone now. Music and videos (non-personal ones anyways) are easy enough to replace, but all the photos and personal videos are gone.
And you're half right when you say that RAID is not backup. But I backup to my NAS, and if it's not a RAID NAS, then if that drive goes, so to do my backups.
Now, you can say that USB doesn't perform as well as a SATA connection right on the MB, or that the connection reliability is not as good. These may be true....depending on your use and expectations. But, ANY mechanism that helps protect data (I don't care if it's connected via a parallel connection or a 2400baud modem) is not a bad thing. If that is the only thing available, why would you block it's use? As long as the expectations of speed and reliability are spelled out, how could making USB drives available for RAID hurt?