Of course another factor not taken into account is that, on a lightly loaded aeroplane, the computer seats pax for fuel efficiency (ie a slightly nose-up attitude). If the pax rearrange themselves willy-nilly, the aircraft might then (in an extreme case - some airlines run on very low budgets) have insufficient fuel reserves to cope with a diversion!
Although we're not casting doubt on passengers intelligence -(please don't confuse "intelligence" with "common-sense" as many people possess the first, but surprisingly few display signs of the latter!) - please give us the credit for knowing more about the business of aviation - it is, after all, our chosen career!
boyo25 - I agree. I have never known a refusal to re-seat a passenger, but, on BA "mainline" metal (dunno about BA Cityflyer or the franchises) it is always recorded somewhere!