I used an iPad at the gym while doing cardio. Have you ever try putting a laptop on an elliptical or treadmill machine?
Also, when I fly in China they make me turn off my iPhone. Flight mode wasn't acceptable. A laptop also takes up a lot of space and really is not convenient when you need your tray for food and beverages.
Why do you take an iPad to the gym?!


Also, a 11" MacBook Air is more than portable IMHO. The usability is definitely outweighs a little bit inconvenience.