What's wrong with just turning up on the day and sitting in an unreserved seat? And if there aren't any, then use your reserved seat.
The unclaimed seat then appears to be unavailable to unreserved passengers, who are left to look for (often elusive) empty unreserved seats, or sit down and take a chance that the person who reserved the seat doesn't turn up.
I sometimes find this a to be an irritation when I (as a season ticket holder with no reservation) board a busy train (e.g. the 1945 from Paddington). I have to stand until after departure and then go and see which seats are unclaimed (and there seem to be an awful lot of them!)
I suppose the person who chooses not to take up their reserved seat could always remove the label to release the seat for use by other passengers, but that's probably against some rule or other.