It'd be from a price/performance perspective.
And it's not exactly right. You could have a movie that's watched frequently, but video files require about 1/15th the access speed of a normal HDD if played normally.
On the other hand you could have a frequently application that has a .INI file which is updated every time it starts, stops or opens a file. But it's a small file and wouldn't really matter if access speed was 40 or 400 Meg/sec.
I think the whole point is to devote SSD to things where speed would benefit greatly. But generally an OS drive will need most of the files for the OS to be resident. The bulk is in stuff like Windows, Prog Files, paging, hibernate, system restore files. Except for stuff like system restore and maybe hibernate you'd want the remainder of that to be quick access anyhow. Docs and Settings is a mixed bag, the general idea there is just leave it as is but keep folders on normal drives for big stuff.
SSD can also be useful for large temporary/work files. e.g. a few Gig worth of video footage in seperate files that you want to edit into a presentation would be well copied to SSD for that process.