RISC OS does do disk cacheing, it's just not the best in the world, that's all.
Windows does not access the disk for fun. Under VRPC on a correctly configured machine, one would expect the disk access to be little more than under RISC OS on an ARM based machine.
However, due to the more modern cacheing algorithms on the PC side, and the fact that they interact with the cache on the drive so much better than the RISC OS implementation (think of the temporal dynamics of the drive what are in the various caches, and how this relates to read-ahead caching and the speed at which a machine can empty that cache), one might propose that an action such as loading an application like OvationPro under VRPC actually subjects the drive to more physical stress than under RISC OS.
Granted if you go back to Windows and load a 200MB graphic into PhotoShop on a 128MB machine it's going to thrash like hell, but doing the same in PhotoDesk would give similar results.