All valid reasons so far. Aviation is all about mitigating risk and trying to keep the chance of avoiding the worst happening as high as humanly poss. Lots of things are like ETOPS, defined by mathematical equations that someone smarter than me has come up with. With this in mind leaving with one pack is perfectly possibly but it degrades your protection. Time of useful consciousness calculations may well come into the equations as FL250 is the cut off for aircraft operating without drop down o2. Just i thought.