We have not flown since 2010, but flew regularly before that and never ever paid extra to book a seat on any scheduled flight, and even on our low cost Jet2 flights to Spain we never paid for seats, and always ended up sitting together, although possibly because we noticed that their on line booking system would allocate sequential seat numbers if you did not pay for seats.
So I guess since 2010 things must have changed, clearly not for the better.