A MacBook air simply has the capability to do more in the same amount of time. It isn't so much of a linear experience, and as such, it all comes down to needs of the user.
I have both, and I only use my iPad for casual stuff, or anything that doesn't require multi tasking. I can't stand trying to manage several Internet tabs with word processing apps, going back and forth, having some web pages not work properly, etc. a Bluetooth keyboard does help wonders with the iPad though. Even YouTube isn't as enjoyable for me because it's so linear. I like to browse in a tree fashion... Always branching out several tabs from one page, closing a few, etc.