Question about the article, and suggested use of hierarchical file storage: why would you want to make the user "mark" files? Why not just have the OS keep mo

Question about the article, and suggested use of hierarchical file storage: why would you want to make the user "mark" files? Why not just have the OS keep most-recently-used (perhaps weighted by size?) files on the fast disk, and migrate inactive files to the slow disk? Or, just buy a hybrid disk and let the hardware do it.