In my view the arguments for dispensing with seat reservations are;
They can lead to an increase in passengers without seats due to uncertainty as to whether a seat is free or reserved (especially with electronic reservation system which do not give full details of any further reservations for a seat)
They lead to congestion when passengers board at stations as they are looking for specific seats rather than just the first available seat.
They can lead to conflict when people are not aware they are sat in a seat reserved by someone else.
They lead to passengers ending up sitting in seats they are not happy with (ie facing the wrong direction)
But these are minor irritations or brief problems: the only real recurrent problem listed here is the failure of electronic reservations on trains
The pro points for seat reservations are;
Groups are able to sit together
Those less able to stand can be guaranteed a seat (but as we know this is often not the case)
More often than not, seat reservations work: we remember the journeys when they don't because those are more memorable. On the majority of services, where people happily sit in their reserved seats, we barely notice their existence.
Anyway, the largest pro point for seat reservations is that they encourage people to use train. Without them, there are many journeys where people with luggage, babies, mobility problems, or in large groups, would chose other forms of transport.
There are clearly some problems with the current system and I'd support the idea of a small charge for reservations. But nothing anyone's said here is a real argument for their removal.