Why would anyone ever want to carry around a two pound battery pack that uses disposable batteries to charge their MacBook Air, which barely weighs more than that by itself? There are reasons, and they may even be good reasons.
Disposable batteries may sound kind of evil, but there's no way to get around the fact that they're fabulously convenient. They provide instant power in a portable package and you can find them more or less everywhere. Custom lithium battery packs are way sexier, for sure, but when they're dry, you're pretty much out of luck unless you can find an outlet and then stand around for a while watching a blinking LED.
This is why adapters that let you power your stuff with disposable batteries are invaluable accessories, especially for travel. When you run low on juice, you can just stick more batteries in, and if you run out of batteries, odds are you can buy more somewhere close. Plus, since disposable batteries are modular, with different adapters you can use the same batteries to power anything you want. The other nice thing about the disposable battery form factor is that you don't actually have to use disposable batteries; if you just need some extra power for the day, use rechargeable instead.
Bird Electron's battery pack for the MacBook Air uses eight "C" batteries to get your skinnily overpriced computer an extra two hours of battery life, which is an extra two hours of sitting in a coffee shop looking smug and hoping people notice how silver and expensive your laptop is. It's $300, batteries sold separately.
Via Akihabara News