The plugin allows to use service accounts only when running in Kubernetes. However, service accounts can be used to access a kubernetes cluster from outside as well. See [1].

To connect a Jenkins instance to an existing kubernetes cluster, it would be very natural for an admin to create a dedicated service account, assign it to a namespace and make use of it to configure the kubernetes plugin.

Since the functionality is already there, I guess the effort would not be to high. Or is there any reason not to do so?

[1] http://kubernetes.io/docs/admin/authentication/