It turns out that one of the apps people really need to run multiple instances of is Windows itself. This is largely Microsoft's fault for bundling every app including the kitchen sink in the OS platform itself. As a condition of using their clean little high-performance kernel, you had to accept a web browser and home-user-friendly userspace.
Little surprise that people are kicking the whole package off of Ring-0 and substituting something like vmware for their $five-figure server hardware.
It's that super-isolation that actually allows multiple apps/roles/data categories to finally share the same hardware.
This tradition started ~98, with Microsoft. Before that, when servers were Suns, IBM and Digital, every server had lots of roles.
Somehow, microsoft convinced the world that it's better to have one server per role (and pay them for some more licenses).