Airlines may not follow great circle tracks for the reasons I listed. North Atlantic tracks are designed to accommodate the huge number of aircraft flying between Europe and North America. With no radar monitoring possible, separation has be be greater. Only a certain number of aircraft can be on a track at a particular time therefore a number of different routes have to be drawn up and aircraft have to comply with minimum navigation performance standards. The tracks chosen take wind into account which obviously differs from day to day.
Free flight is simply not possible with the ATC system at the moment, aircraft have to follow specific departure, enroute and arrival procedures for seperation to be maintained. With advances in technology, free flight may be the norm in a few decades time.
On the Pacific ETOPS comes into play and routing may differ depending on the airlines approval time or weather conditions which render an ETOPS alternate unsuitable during the required time period.
Singapore Airlines had a number of widely differing routes on their business class A340 service to New York and chose the best one for the day.
Auckland to London gives the choice of going east or west, Air New Zealand are a true around e world airline.
I have nearly thirty years of flying as a pilot rather than a passenger.