The number of reservations that are never used is verging on scandulous.
It is about time that franchises that use them should charge a small fee, say £2-3 per reservation to encourage people to get the train they intended, or sit in the right seat.
It's more a function of the way people buy tickets online and get a reservation by default. So blame the system, not the people. Though I agree that a nominal charge would be effective at reducing the problem I suspect it is one of those issues, like charging for bikes on trains, that is deemed unacceptable to try and implement, even if logically there is at the very least an argument for it.