Although it's important to have that level of freedom, it hasn't always been the case in the PC world. Furthermore, there are OSes out there working on PC which

Although it's important to have that level of freedom, it hasn't always been the case in the PC world. Furthermore, there are OSes out there working on PC which don't have that level of interop. Finally, some people don't have 2 different OSes installed, and if their setup breaks, they'll surely need another box to fix it. OTOH, needing a second machine for that sort of maintenance (installing a second drive iiuc) seems indeed quite a big requirement.