As a single traveller I get annoyed when I have to pay a single supplement.
I have recently noticed adverts for cruises where the second person goes free.
What is to stop someone signing up for a cruise like this and getting, for example, my sister to allow me to add her details as my 'second person' and then just not having her turn up, due to example, a mild illness?
I would get a nice big double stateroom and would avoid the dreaded supplement.