It's not entirely generational though. I use an iPad for a lot of stuff even with a laptop available on my desk. The reason is that I'm a power user, and on my laptop I'm too likely to start fiddling with stuff and get distracted (might be a mild form of ADD).
And just to be clear, I'm a serious power user. I use hammersmith, BetterTouchTool, vim keybindings wherever I can, and Alfred to the extent that almost any key is involved in some kind of shortcuts. I avoid emacs precisely because I'm afraid I'll get even worse.
The fact that on the iPad I have to focus on the article I'm reading, the the email I'm writing, or in general the app I'm working on, well, all that helps me to stay focused.