I think what you're referring to here is the 'free seating' policy that most UK users of the 737-700 have in place.
If the load is less than full, we like to spread the load throughout the cabin evenly. This is because the trim setting on take-off is calculated based on an even spread of passengers.
If we do not do this, on take-off, the plane will be either nose heavy (too many pax at front) or tail heavy (too many pax at back). This means the plane will either be very difficult to rotate at the correct speed or will try to rotate too early, depending on the loading bias.
It's not so critical on landing as we trim as necessary to keep the airplane at the correct attitiude and speed, providing the load is not out of CofG limits of course.
All airlines that have allocated seating calculate the correct trim based on actual position of the passengers, therefore negating the need to reposition pax once on board.
The whole scenario is not really that crucial on a shorter airplane like the -700, but certainly is on the -800/-900. I would imagine any airlines operating the free seating policy on the 320/321 have the same concerns.
DTD.