My experience in the design of these is on the cracked gas processing side for naphtha crackers.
However, it is well known that fired heaters for cracking naphtha or gas are in very severe thermal and cycling service. So if you lose the spare, actual annual production will drop when one of the heaters is on maintenance (very likely) and there is no spare. So then you can only commit less production to your customers. Though I agree that the fired heater is a high capital cost item.
Some one in your organisation can estimate whether the net gain in production by keeping the spare can offset the increased cost of the fired heater over the life of the heater ?