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?
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.
Regarding local councils subsidising off-peak bus services, I agree very much with you that public transport should be seen as a community service, but, unfortunately, in the present economic climate, those subsidies are likely to be withdrawn as councils make hard decisions over the way they use their limited funds, and evening and Sunday bus services begin to disappear again.