I really wish they didn't include this. Whilst kustomize is infinitely better than helm neither are good tools.
That said if there was a choice between the 2 kustomize wins hands down I guess.
I am, however, not sure what kubecfg's future is: ksonnet (its way more complicated cousin) is being deprecated.
Heptio (the owners of ksonnet) were bought by VMWare and will no longer maintain it. But it's possible the CoreOS guys will take ownership since they use it heavily in some of their tools.