Matthias,
Network login is a security feature thought to secure access to the network from ports accessible to normal employees and visitors, to make sure nobody can gain access to the network by simply plugging a device in an empty port.
Servers, on the other hand, tend to be grouped in protected environments (datacenters) with ports not available to visitors or normal employees. Datacenters have their own security measures that don't include networks login.
I imagine that network login would be disruptive in the current virtualized datacenter, where VMs can be moved from one physical server/network port to another without the VM knowing it is being moved. Because of this, the datacenter network has to include tools (e.g. Data Center Manager) to make sure that the destination port has the same configuration as the original port. If the VM is unaware of it being moved to a new port, how would it re-negotiate access through Network Login?
I don't know how easy/difficult it is to enable this, but you can always work with GTAC and your local SE to make a feature request.
