Soldato
I have heard that sometimes it can be a bit of a hassle trying to fly with a one way ticket.
Maybe if you are flying long haul to an unusual destination and you either look like a vagrant, or have so much luggage you may not leave. If you are travelling to somewhere like France from the UK with a UK passport they aren't going to be asking you about a return. If you go Eurostar one way from London they ask you nothing at all, ever.