I never understand how these are unused. Do people book a ticket and reservation and then think nah I'll get a different train?
I've done this. As long as reservations are not chargeable people will do it - just reserve on their most likely train.
Or do they just get on and not bother with their reservation because its empty anyway?
And this. I like seat selectors, though, as they allow me to choose the seat I want (or to know it is taken and choose a different train or flight if possible). Prior to these a reservation was more of an "if it's busy I will use it, otherwise I will choose my preferred seat".
As opposed I suppose to the OCD reservation hunters. Like I said before, had someone once who walked up and down the train for 20 minutes trying to find their exact seat (and tried to incorrectly twice turf me out of my seat). This was on a train about 10% full!
To be fair to such people, occasional users don't know you can sit where you like, nor do they know about reservation marking. Like how you pay the fare on a UK bus, it's something that is never actually written anywhere, you just have to know or ask someone.
It's quite a reasonable assumption that if you have a ticket with a seat number on it that you must sit in that seat, even if it isn't the case for most TOCs. Indeed, the wording of the Advance T&Cs strongly imply (at GNER's behest, if I recall rightly) that you must sit in the booked seat.