I don't think anyone is actually denying that the OS does lots of little random reads and writes in the background--especially modern OSs. All they do is process lots of small data without the users knowledge. That said, there's a difference between accessing a few dozen files every second, and accessing tens of thousands of files every second. Remember, the OS might do lots of small random writes, but in the end, it's still designed for HDD storage, as 99% of users still use this. So, an OS will NEVER demand more than what a HDD can do. Not while HDDs are the most popular non-volatile internal storage media. So let's just close the book on needing an SSD for OS background tasks. Very helpful, but not crucial.
I think what the recent news in SSDs suggest is that what vendors understand is that, to a user, there's no tangible difference between a drive that does 50MB/s random 4k writes and a drive that does 150MB/s random 4k writes when all software is designed to run on HDDs that only manage a 1-2 MB/s in 4k writes. That's why every SSD vendor has decided to make the big gains in sequential transfers. Not that it's more important, but that it all they can do to make SSDs feel faster.
Probably, but the difference between a RAID SSD setup and a single SSD setup is not random performance--It's strictly sequential. So If they start copying video files and such, they'll surely notice a difference. But they won't notice one in common file loading. Not one that is tangible, anyways.
Again, I'd never argue the merits of getting an SSD in a system, whether it be a single SSD, or a RAID config. But, all it takes is one SSD to get the relatively instantaneous access times that make them awesome. Any additional SSDs are only padding your sequential throughput.