Security. You ever run a process in its own VM for security reasons? A hypervisor is a microkernel with bloaty processes and even more hardware protections.
Monolithic kernel = all familiar kernel abstractions (processes, IPC, filesystem, virtual memory, device drivers, networking stack) are built into the kernel.
Microkernel = Most kernel abstractions run as separate processes. Kernel communicates with them over IPC.
Exokernel/unikernel = There are no kernel abstractions. Instead, these abstractions are built into runtime libraries that are then linked into applications. The kernel's only job is to securely multiplex the hardware, and it exports an interface that is similar to bare metal. In 2016, the exokernel is called a hypervisor, and the libOS is called a unikernel.
Neokernel = the programming language's runtime library provides the primary kernel abstractions, along with any interprocess security. Machines are limited to a single programming language.
Most people have experience running processes in isolated VMs for security reasons. I was trying to use that as an anchor point to explain why one might want to have components of a single operating system run as separate servers, for much the same reason -- compartmentalized security.
Most people run VMs for hardware utilization purposes.
There are a number of ways to build unikernels and they're not all OCaml (which is just MirageOS) -- http://unikernel.org is trying to bring these together.
[1] http://roscidus.com/blog/blog/2016/01/01/a-unikernel-firewal...
It would be great to get a list of live unikernel deployments listed somewhere to address questions like this when they come up.
Hipsters.