BA started charging for seat allocation around 3 years ago I think. First is the only class that doesn't pay, but I stand to be corrected, as I only use CW and CE.
It seems less the cost of the seat reservation for the infrequent flyer than a perk they want to provide their Gold and Silver status pax--because status pax can choose ahead of OLCI without paying the fee.
The first time I paid $3000 for a Club World seat as an upgrade, then had to pony up for the $90 seat reservation fee for each leg, it was a bit of a shock. But after doing this now for on or about 3 years it's become the norm--for better or worse.
What this has done is for the moderately frequent traveller without status--who now knows that if your route is popular and you don't want to get stuck with the worst seat on the plane in your cabin at OLCI--you pay the fee and move on.
This pay-in-advance policy and the many who opt for it, has made FAR fewer seats available at OLCI for those who won't pay the fee--because the regular travellers know on their very busy routes not to wait for 24 hour pot luck.
Then again, many travellers don't care and just take their chances at OLCI and do fine most of the time if their routes are not jammed with full cabins.
But just one trip where I did not pay and was consigned to travelling in the worst seat nearest the loo and noise of the galley and I was convinced of shelling out the extra $90 and it seemed well worth the price.
And it's $120 to reserve a seat on the upper deck.