When I've booked flights, on every single booking I've done, with Southwest being the exception, I've bought and paid for particular seats with my ticket purchase. Sometimes there's a need for a change when boarding, and the boarding pass will be changed to reflect that, but most times when I did fly, the ticket and boarding pass both reflected my seat assignment.
I say this noting that the last time I flew was in March, 2014, and prior to that would have been July 2010. I'm not keen on dealing with the bullsquat of flying commercial these days.
And Southwest is the best exception. You get to choose your seat as you board. Not at the head of the line? You can pay at the gate to board first in "business class" by paying an extra fee ($40 I believe). Of course you may still have a 250 pound Bubba take the middle seat, but you take your chances with any other airline in that regard too. I was to fly Delta in March; my first non-SWA flight in many years on a budget fare. Unfortunately, my travel date needed to change and the cost of the new ticket ($200 change fee plus difference in fare) was greater than the original ticket cost plus my new SWA fare! I chalked it up to lesson learned and now stick with Southwest only.