The only reason to do this- despite the wastefulness- is to buy time. Given a single window a/c unit, the op says there's a backup unit- which is likely not running all the time. When (not if) the first unit fails, the temp will climb very rapidly- once the room crosses the 85F mark, you're counting minutes before thermal shutdown starts to happen. In my room (15Kw ave consumption, north central US), keeping the room at 63F gives us exactly 15 minutes to get our backup cooling online- reasonable if we get the notice early enough, and are on-site. Raising it to 73F would give us 7 minutes or so- that's a dangerously short amount of time.