At the risk of sounding terribly naive, whats the purpose of a MBA & an iPad? I always figured I'd have a desktop & air or an ipad and MBP (my current setup). I would want to retain a device with plenty of space, but it just seems redundant to have two very expensive ultra lightweight devices when likely neither has enough space to be "home base" - and I really can't imagine having an iPad, air AND a desktop. One device that won't weigh down my bag too much seems like plenty...
Actually, the 13-inch Air, especially with maxed out specs, has plenty of "space" -- by which I suppose you mean hard drive space? But even if you mean things like RAM and CPU power, the Air is plenty powerful enough for most usage. For all but the most hard-core power users, I'd say the Air is a perfectly capable "home base" computer. And if you need more hard drive space, or you want a larger monitor while at a desk, you can always attach external drive / monitor.
Now if you already have a MBP, of course, there's no need to switch to an Air. But if you are considering buying a new notebook, then IMO, there's no need to go for a MBP unless you really need super computing power. And if you plan on carrying your notebook with you on a regular basis, instead of just using it as a desktop replacement, then the Air's thinness and lightness is a huge advantage over the MBP.
As for why some people might carry both an Air and an iPad, well, they are just good at different things. Like, if you have lots of PDF you want to read and annotate, then the iPad is better than the Air. But if you need to type out a long essay, then the Air is probably better. Not everyone needs to do both on the go, but a person who does need to do both might decide that carrying both is the way to go for him.
