Agreed.Again, fair enough on a busy services where there is a shortage of seats, but totally absurd when the carriage is half empty.
Indeed I'd go as far to say it's inconsiderate to choose to sit next to someone when there's an empty pair of seats anywhere in the coach/train (I often walk the full length of a 390, finding coach U rammed and coach A half empty - although I wouldn't expect everyone to walk the whole length of the train of course!)