The original NT was designed by the same chap who designed VMS. It was a rather elegant microkernel. However, for performance reasons, more and more found its way into the kernel.
flypig's point about resource contension is a good one. Most modern UNIXes make heavy use of buffering and caching, as well as sensibly designed file systems to make the problem all but go away. I'd hate to think of the performance of the RISC OS file system, all the way from the hardware drivers, to FileCore, to FileSwitch, should true PMT happen, and more than one process want to access the disc at the same time. I suspect it'd suck big time, as none of them are designed for it in the slightest.