s j w: I disagree on all 3 accounts.
1. I think its pretty rare to break a screen, but with that said most ultra portables I have come across use a lot more metal in their construction over plastic; so a stronger frame and less weight means less likely to be damaged. Plus to get the small size they are moving to more solid state components, such as SSD's, not having an optical drive, etc... So you have less components to be damaged and the ones you do have tend to be more durable.
2. MacBook Air come as low as $999 a dicent build is still under $1500. A good full sized laptop is just as expensive.
3. I will break this down into sub groups
a: I completely disagree about Laptop's being secondary computers. They are powerful enough for most users to use them as their primary, having 2 computers cost more in the long run (from a business perspective) as then you have twice as many software licenses, twice as much hardware to maintain, etc... We have many people at my company (myself included) where a laptop is our only computer.
b: I will agree about needing a safe storage device, but with SSD's the likelihood of failure is minimized, and you couple that with a online or network biased backup (such as CrashPlan) and the need for a desktop is gone.
c: Because of the two above reasons I disagree about being overpowered, and to some extent over priced. If you are using these as your primary computers (as I am sure many are) you don't want an anemic CPU such as an Atom. And they are not as overpowered as the the i5 or i7 name implies.
i. They are only dual core
ii. Their base speed is much lower, they can boost up to higher speeds but they don't stay there.
iii. They have smaller cache memory
iv. Their cost to performance ratio is not as good as a desktop, but battery life is worth a lot for a road warrior, and although many might not need an 8 hour battery, it is nice to be able to have one.
As an I.T. person for a medium sized business I can say these facts about laptops:
1. If its heavy, the won't take it on the road, and that decreases productivity and higher turn over.
2. If its slow (not as fast as "their computer" at home) They will whine until someone gets them a laptop that meets their expectations. I can handle whining, but it seems upper management cannot so you will end up getting one anyway.
3. If it has a poor battery life, you basically have to buy them two power adapters. One for their office and One for their laptop bag. At about $100 for a laptop power supply; that instantly takes a big chuck of your savings on a cheaper CPU away. BECAUSE THEY WILL FORGET THEIR POWER ADAPTOR AT HOME.
I can seriously make it an entire work day on my MacBook Air's battery. I haven't needed to, but sometimes its nice to just plop your laptop open and start working.