Students and teachers are the target audience. Salespeople are. Business people are. These people all have serious computing needs that are better met by an ultraportable device with long battery life than by anything that has more horsepower or more ports.
Casual Facebook users are not a target audience for anything, because almost everybody is a casual facebook user, including people who do 3D animation or need to compile huge codebases.
The grandparent post was talking about all-day battery life, not about horsepower or ports.
But I don't see that being realistic, yet, because (a) the new MacBook has less battery life than the 13" Air, (b) OS X is messier than iOS (system daemons love to run at 100% CPU once in a while), (c) as you said, almost everyone is a casual Facebook user and runs terrible crap like Flash. (iOS is at an advantage here again)
But yeah, it is a great device for people who only open their laptops occasionally and can recharge at night.
With the limited number of charge-discharge cycles that even LiPo batteries have (typically estimated at ~500), that means you'd be replacing the machine within its depreciation lifetime if you fully — or even mostly — discharged it daily.
Macbook batteries are rated at 80% after 1,000 cycles. And battery replacement on an MBA is a $129 ($199 for an MBP) service that they can do at the Apple Store while you wait. Lenovo wants $139 for a replacement battery for a T4xx of similar capacity.
I don't understand the problem, really. Apple WILL replace your battery. It's not like you have to throw away the laptop once the battery is dead. It costs £99 in the Apple Store, which is actually less money than a new battery for some Dell laptops costs.
I'm seeing reports of Apple laptop batteries barely lasting an hour after 400 cycles, having 75% of their rated capacity after 900, and having 50% after 1100.
I think usage pattern probably matters far more than the strict cycle count. Specifically, "all-day" usage appears to correlate strongly with battery capacity falling off more sharply as cycle count increases — admittedly, based on user-reported behavior that I could find on the web with as much time as I can spare this afternoon to faff about on the matter.