First, you don't get 9 hours doing something more intensive, even watching video. Second, with a mobile device, its a pain to always have to remember to plug in when you have the opportunity. I have power outlets in my house, but that doesn't mean I want to be tethered to a cord when I'm working on my couch.
The way I use my iPad or MBA is to throw it in my briefcase and use it intermittently and ideally not plug it in for a week. The better the battery life and the lower the idle power draw, the less likely it is that you'll have to find a power cord when its inconvenient.
The way I use my iPad or MBA is to throw it in my briefcase and use it intermittently and ideally not plug it in for a week. The better the battery life and the lower the idle power draw, the less likely it is that you'll have to find a power cord when its inconvenient.