Pax dispersal on flight - saving fuel or ???

[bananas_oz]

I have now been on several flights with less than 50% load and have wondered about different airlines responses to this in terms of loading the aircraft. Some examples: A340 pax evenly distibuted, 737 pax all over wing roots, A320 mostly in the middle 3rd, etc.
Some possible reasons for this are to balance the load of the aircraft, save fuel (possibly linked to no 1) ease of service by CC, reduction in CC required, though for multi legs this wouldn't work.
There obviously is logic to this, just wondering what it is.
Cheers.

[BOAC]

In my experience (737) very little effort goes into deciding how a part-load of pax is sat. Generally it is 'stick them in the middle' which will not adversely affect balance, but it should be remembered that baggage loading may disrupt that 'careful plan'. Part loads can be tricky to seat, but since there is an ever-present ability of check-in/dispatch to seat the wrong pax at overwings/emergency exits (children, disabled, etc), I do not think we will see any 'logic' in loading part-loads. Remember that by the time it gets to producing a 'balance' on the load-sheet the seat allocation is well past, and trying to re-arrange that causes upheaval.

[luoto]

There is also the customer services aspect to consider. Alienating regular passengers who have pre booked / assigned with online checkin seats. There is a perceptible difference between justified operational needs (mother needing bassinet seat or even maybe CIP/VIP or inop seat and just to save a few kgs of fuel. IMHO. It might be short sighted to pee off the passenger if they've gone from a nice aisle seat to a sh1tty middle seat at the back with no recline.

I know that, since it happened to me in a previous "life" and KLM/NW got pushed down the preferred routing list from several flights a week with that particular carrier.