The seats used to be a cheap way of getting to Fort William or Inverness - at around the £40 or £50 mark each way, they cost the same or a bit less than the best you can generally expect to do with an advance ticket booked some time beforehand. And while you couldn't be guaranteed there'd be any on a walk-up basis, mostly you could get one at moderately short notice.
The pricing has now changed though, and Inverness at least seems to be £75 for a seat. While that's not ludicrously expensive, and it's still a bit less (per leg) than a super off peak return, it's no longer the cheapest option, as you can often do better than that using advance tickets on day trains.
For me it's swung my preference towards just getting the day train if I want to do the journey at reasonable cost. Doing the seats (for me at least) partly wipes out the next day thanks to not really getting any sleep, and that was worth accepting in exchange for saving some cash, but if I can just travel on a day train, for less, then the argument for the sleeper seats is considerably weakened.