The family with small kids flying to Malaga for their annual holiday gain significantly from a move to assigned seating.
The solo frequent traveller who lives near a major airport, flies with only hand luggage, pays for the ticket from their own money (i.e. no expense account claim) and who doesn't have any special mobility needs really loses out with a move to assigned seating. Being a frequent traveller in a major airport, he/she may use a variety of airlines, so doesn't see a benefit from a Plus card.
Apart from when a flight really is full, there always seems to be a sole unoccupied middle seat near the front exit regardless of how late you board, which makes it easy to get off the plane quickly, be near the front of the queue for any kind of immigration control, whizz through customs (no need to wait for a baggage carousel) and get out of the airport and onto a final destination as fast as possible.
Being assigned a seat at the back of the bus would *really* annoy this kind of passenger.