In the early game, being even somewhat tough, immortal and flying would be a big bonus. I'm not sure if they are ethereal, but almost any weapon/weaopn+shield should be enough, and their fear won't hurt... And there is always the chance of finding Bowl of the Lost of a Demon Gate.
Yes, they are ethereal. Their usefulness is also dependent on research difficulty. Hard or very hard research means that vampire counts come in real handy for BF Ulm. All they need is a few blood hunters and you can get a VC about every two or three turns.