Well, for starters, a replacement battery for a MacBook Pro also costs $129. Admittedly, the MBP's battery is more complicated, since it has a nice metallic enclosure and a button with little LEDs that shows you how much charge the battery currently has (which is a GREAT feature, BTW), while the MBA's battery is likely just some cells wrapped in a plastic membrane with a small power lead running off of it. But then again, that $129 includes the cost of installation (labor). So I think it's an even trade-off.
As far as asking to share someone else's CD/DVD, the problem there is that that person has to install the Remote Disc software... which comes on the MBA's installation discs. If you don't have them with you, or if the software isn't available from the Apple web site, then they wouldn't be able to share their drive to begin with.
I would imagine that Apple did some surveying and usability testing and figured out that ultraportable laptop users use their CD/DVD drives "x" percent of the time, where "x" is a small enough number that the advantage of not being burdened with the additional weight of an optical drive grossly outweighs the disadvantage of not always having it with you. And for those times when you do need an optical drive, there are software (free) and hardware ($99) solutions available.
With the battery I guess im also factoring in the annoyance of having to potentially lose your laptop just to replace the battery whereas in the MBP I can do it myself.... altho im not sure if the replacement is done in store, but if it is I know 2 stores in my area where i would trust them to even open the box to the battery.
Also, im sure that the remote disc software is going to be available on the site, but that is besides the point, it was just a off comment that I would find it funny if that ever happened. What makes or breaks a lot of products for me is those off occasions. Those times where for some reason a program isn't running right, so i have to reinstall.... or I'm at a clients house and they don't have wifi and hand me a CD of next weeks target graft sales. Both are VERY rare, but happen... even if it happens once every 6 months it happens... and its important. Like I said, the MBA isn't for me, but I know these forums, and I know the moaning that will eventually happen. I mean damn.... look at all the complaining even tho not a single person here actually has one in there hands.