Having just got back from Birmingham I have not read all the posts here yet but I do wish to reply to some I have read later.I believe it to be the case that the printer at Waterloo was broken half the time/people were forgetting to put them out when it was working. At one point, they started rear loading them from Weymouth for the SWML services.
Seat reservations are not necessary for short bus journeys, where it's not too inconvenient to stand or there may be another along in a few minutes. But for longer distance journeys, e.g. National Express coaches, IIRC the driver is provided with a running tally, allowing her/him to see the expected loadings throughout the trip, and will only allow "turn-up and go" passengers to board provided there will be room for those who've pre-booked and may be boarding later.I put it to the 'cour't that buses don't have seat reservations so why should trains?
In fact why not pose the argument that buses are often subsidised outside their peaks and sometimes run empty (whilst still providing an essential link sometimes) so trains might as well run without passengers on them - after all it keeps the running costs down?