In most applications of graphs, links make no sense without nodes (they are defined as an ordered pair of nodes in the mathematical graph model) so they are removed by default. You might try setting a link's Origin or Destination to null before removing its respective node so that the link remains in the diagram. The control will actually associate the link with a DummyNode object in this case, and also when AllowUnconnectedLinks is enabled and the users draw an unconnected link. So if you need to find out whehter a link is disconnected, you must compare the type of its nodes with DummyNode.

I hope that helps,
Stoyan