Other reason for having the OS on SSD is that one doesn't frequently change the OS data - it's mainly read, except for times one does an OS update. Given that the endurance of SSDs are lower than that of HDDs, it makes sense that something that's less frequently updated would sit on an SSD, while user data, which is frequently updated, would sit on an HDD. Also, the OS is a fixed size, and would typically be something like 16GB. So one could get an SLC SSD - one w/ the highest performance, w/ a PCIe interface, w/ a low enough density to avoid blowing up the cost of the configuration, and just use it for the OS.
Also - how big is the OS itself - in terms of Mb/Gb? Windows 8.1 is overall some 16GB, from what I understand, so how big would the kernel be? Reason I ask is that so far, PCs have had a NOR flash of 4Mb for their BIOS. Given how memory densities have grown, there is the scope here to grow the BIOS flash and contain within it the entire kernel of Linux/*BSD/Windows8.*, and then let the rest of the OS reside on the storage.
That way, the system is more secure, since there are ways a BIOS flash can be protected by hardware (Write Protection states, for one) and other things that fall in Userland can go into an SSD or an HDD, depending on the system configuration. User data, such as movies, can go into the HDDs.