So usually if a device is offline, the neighbor count will slowly drop to 1 (hub only). For some unknown reason, it seems like they sometimes get stuck and the other devices refuse to forget about the neighbor. From what I understand, in this case there is a chance that when other devices are re-routing they would possibly attempt to route through this dead node. For a dense and healthy mesh, it should not really be much of an impact, especially with only one of these "ghost" nodes.
To get rid of it, unfortunately the hub will refuse as it has been recently found that is neighbors are showing the hub will always say the node is no longer in the failed list when you try to remove. You could try to refresh until you get the remove button, maybe it will work, but probably not. The best way in this case is to use a USB stick as a secondary controller. There is guide here: How To: Remove Z-Wave Ghosts (including using a UZB Stick)
