Once the decision has been taken to remove functionality from the kernel and place the functionality of the operating system into a user-level task, the next natural inclination is to split the kernel up into separate bits. This has the advantage of shielding one portion of the kernel from another and allows for replacement of part of the kernel while leaving other portions undisturbed. It can also allow for operating systems with vastly different requirements to run on the same machine. For example, a real-time scheduler can coexist with a typical Unix scheduler, allowing non-critical tasks to run when the real-time systems are idle.
There may be additional overheads involved with this approach. In particular if task switching is slow, communication between the different servers involved will be slow.