In the broad sense I think you could consider it an HTPC, but I would consider it to be more of an embedded device/appliance by virtue of running Android.
I can see some potential in a system based on this, especially if Android TV ever ends up supporting CableCard DVR functionality. Actually, if that happens, I would imagine WMC use would see a precipitous fall, and a lot of Cablecard users who are tired of the quirks/limitations of WMC would flock to an Android solution.
An Android DVR that supported Plex/MB3 as well as NetFlix/Hulu/Amazon and network apps like HBO and ESPN? That's a home theater enthusiasts wet dream.
If Android TV did get that functionality, I would expect a lot of enthusiasts to implement home built solutions, but I would expect some companies out there (Roku or even SiliconDust) to roll out complete plug and play solutions eventually.
Without Live TV & DVR functionality I think it would basically be competing with Roku devices, and I'm not sure what there would be to gain by going with a home built solution.
RAID protection is only for failed drives. That's it. It's no replacement for a proper backup.