The big advantage of the sleeper for me is time. It takes all day to travel to Inverness or Fort William on day trains but the sleeper allows two extra days holiday or less time away if on business.
I don't even consider flying short haul on environmental grounds.
FW is probably the "unique case" because it's such a so-and-so to get to and doesn't have an airport. To anywhere else most people will fly, so people who won't aren't presently enough of a market.
Though personally I prefer day trains as the trip is part of the holiday, and the journey, provided you go West Coast, is interesting pretty much throughout.