"Providing machines for Kubernetes in not only necessary as a pool of raw cycles and bytes but also can provide a critical extra layer of security". [0]
Maybe it's true for now, but containers may be enough security in the future.
The reasons are actually not any different than when running on a dedicated machine; running containers is a good idea, regardless of whether it's a VM. Containerization is a smaller, granular unit of isolation than a VM, and it's complementary to virtualization, not a replacement/competitor.
It's vital if you use a Mac/Win as a dev machine.
@daveoflynn is correct the LXC founding that docker is built on only currently works in Linux.