The MBA has the same footprint.
The MBA is 25% thinner than a MBP 13"
The MBA has <items that are irrelevant to an MBA purchaser>
The MBA is more expensive.
Think this through with me nutjob. Imagine that the two most important specs to you as a computer buyer are weight (which you ignored) and thinness, for which the MBA is a 25% improvement.
The most important thing to you is weight and thinness.
Weight and thinness. (This is a mental exercise) OK so you are a computer buyer and WEIGHT and THINNESS are your most important specifications.
Now you go and buy a MBP 13". And someone says:
Why were you an idiot to buy a MBP 13"? Compared to the MBA it is
33% THICKER than an MBA!
50% HEAVIER than an MBA!
Has extra useless things that add unnecessary weight!
At least it saved you a few bucks...
You would have chosen the wrong computer if you could have afforded the MBA. What you can't seem to comprehend is the features that give you benefit aren't the same features that give other people benefits.
For a MBA owner, the features that provide benefit are weight and thinness. Faster CPUs, more ports, CD-ROM drives, etc are features that provide NO benefits. They actually hinder the benefits that are important to you...weight and thinness.
If Weight and Thinness aren't your primary concerns, then Apple makes a product for you! It's called the Macbook Pro!
If a 33% increase in thickness and 50% increase in weight for a fat, porky MBP isn't worth it, than your priorities are different from a MBA owner.
Finally, your last question:
"You trade off a loss of ports and performance for more cost. Why would someone do this?"
Well. YOU did it when you bought a Macbook Pro. There are lots of notebook computers that have more ports and performance for far less cost. Why did YOU do it? Oh, because you valued something other than ports and performance in your analysis. In this case, OS X. Well, the Thin and Light crowd value that too. They will pay more for it. Now do you understand the answer to your own question?
That's weak. You're taking the two things that the MBA has as an advantage, moderately less thickness and weight and turning them into key requirements. Nice try, but totally unconvincing.
The real question is are you actually getting much portability for what you lose? Over a MBP 13" that's 0.25" and 1.5lbs. You still have to find 0.75" of space since it has the same footprint.
If you just admitted they buy the MBA "to look cool" or because it's pretty I'd understand, but trying to argue it's somehow more practical doesn't work.