I fly the 757/767 series....and routinely turn the Recirc fans off on long haul (over three hours flight time). .. .When my seniority only allowed my to fly USA transcons, I found that I had a lot more "medical emergencies" with pax going on 02 during the weekends. Looking at the pax involved, they were generally obese or old or a combination of the two.. .. .I started running with the recircs off to push the packs to high flow all the time. It does burn slightly more fuel, but one medical diversion can burn more fuel than a lifetime of running with the Recirc's off.. .. .The only problem I've noticed is sometimes the temperature "pools" in hot or cool in different areas of the cabin. A quick blast of the recircs mixes it up again.. .. .The other problem is that sooner or later I'll hit that left recirc on a 757 and lock open the smoke valve....That'll cost me some beer for the mechanics and a dance in the flight office...