Well I've found that a ping scan discovered more items than the XDD did believe it or not... I'm assuming the network switches are keeping some of the broadcast away from the client that's running XDD.
A lot of what I'm concerned with will remain unmanaged. I have about 35 cisco WAP's for instance, plus about 30 Ubuntu servers and another 20 Ubuntu workstations. These devices will never have a client installed, but given that the XDD discovery hasn't found everything that a ping sweep did, I'm concerned that never running a ping sweep might let me miss a rogue device attached to the network. I guess I could always just create a group for the dupe entries and shove them all in it.