Are ghost flights just a UK problem though? The practice seems to originate from the 'parliamentary trains' which operate just because it is cheaper to serve a station once each week than to go through the legal motions of closing down a station.
At MME you even have a ghost airport station if you want to call it that!
I think there should be another category for totally under-used services - I have heard of PSO equivalent routes in the US with average loads of 1 each way. I suppose single digit pax loads on a 737 or larger aren#t that uncommon if they are full in the other direction?