Portability for me isn't just about working away from home but also about freeing me from sitting at a desk too much. I saw this drawing on Facebook a while ago, a skeleton sitting at a computer, the caption said: sitting is the new smoking! It was humorous but not entirely groundless. Sitting apparently does damage to more than your back ... To various organs and increases the risk of bowel cancer, according to a few things I've read, which may or may not be true.
Some people have had desks made to work standing up. Not just recently but long ago, Hemmingway for example. I wouldn't go that far but with the Air and the iPad I have reduced desk time and i haven't had any back problems since then.
I've written about this a few times and I don't wish to make a crusade of it, but for writers it can be a serious issue ... if, like me, you have a tendency to become so immersed that you forget to eat let alone jump up in the middle of a very promising paragraph to rush to a gym or do mind-emptying yoga. Ha ha.