randylud,
I see your point about the "power off" states not being important. Most of the time they are not, but I think it is wise to at least be aware of those other four states. Remember too, I said ".. at least 8 possibilities". What about the conditions when you use subroutines or state relays? Then you could double the number of possible output states to 16!
I do remember several times troubleshooting a circuit in a larger plant, and looking at a XIO bit, and saying, "Hey, that PLC input bit should be off", and then spending a good deal of time checking the logic and the PLC to make sure it was not faulted and not offline, then finally checking the control power for that circuit and finding a blown fuse for that input. With a blown fuse or a disconnected wire, an XIO bit will be on, even if it should be off when the control power is on. (See the last two lines in my table). Sometimes it does matter about those other 4 states.