why is it recommended to use --net=host and/or why is it said not to work with NAT?

I see Kubernetes just rolled out DaemonSets in 1.1 which is meant to deploy pods on specific nodes (or all nodes) and that seems like a better solution since the Ceph cluster can then be tracked through the K8s logging /APIs, while keeping the nodes on the internal network.

However the way this network layer is built, as I understand it, is basically NAT (I use Flannel) so does it actually work on Kubernetes with Flannel?
I don't see a way to set --net=host for a k8s pod.