I run about 90% of the systems I manage in RAID 10 (there are a few oddballs in there, some only support 2 drives, those are RAID 1, and there's a few where I don't care about performance, but do care about drive space, those run RAID 5/6). The real world performance difference of RAID 10 over a single drive is very large. Assuming a four drive RAID 10 array, expect between 2x and 4x improvement in both random and sequential read/write performance.
WIth that in mind, at $dayjob, we run a lot of VMs. Before SSDs were affordable, we could usually fit between 6 and 8 VMs on a single host (with 4x or 6x 7200 rpm drives in RAID 10) before they became unusably slow, with tons of time spent in disk wait. CPU time and memory usage were rarely limiting factors. As soon as we started deploying SSDs, the only problem was running out of space. Right now we have over 50 VMs running on a single 8x SSD RAID 10 array, and it's blindingly fast.
There's a similar story with databases. Back before SSDs were affordable, we bought a machine with enough RAM to keep the entire database cached in memory, as it was just too slow to run off of 15k RPM SAS drives. On a fresh boot, we'd still need to precache the database into memory, and with said HDDs, that's a job that took something like 10 minutes and was almost entirely disk bound. We recently upgraded that machine to SSDs, and the same precache task now takes under 30 seconds.
As for home users, well that's a different story. Personally I think it's downright irresponsible to run any system with a single drive (HDD or SSD), but the overwhelming majority of existing machines with a single drive suggest that my opinions on this matter are not widely held.
I guess my issue with your proposal is that I just can't see very many cases where it's practical. The low end of the market is dominated by Laptops/Desktops/Tablets/whatever that cost under $500 and all have only a single drive, as an extra $100 for another drive is going to be a dealbreaker most of the time (if another drive would even physically fit). The high end of the market where performance is critical, is completely dominated by SSDs. You can read countless stories of big companies replacing full racks (42U) of HDDs with 1U or 2U of SSDs. I guess somewhere in the middle there is a small set of people who:
- store a lot of non-media* files (over 500G or 1T)
- are not overly concerned with performance
- have the technical know-how to set up and maintain a RAID array
- are significantly more concerned with reliability than most
- are still relatively cost-sensitive
Those people would probably be better served by a 4x HDD RAID 10 array than a 2x SSD RAID 1 array.
* If you're storing media files on SSDs, you either have too much money to burn, or zero sense. They're huge and 99% of the time are read/written sequentially.