And one more ugly piece of plastic that many people ruin their Macs with - USB 3G sticks, because no Mac series can hold a SIM card, even though Apple has successfully pushed for a more compact standard.
My battery lasts 6 to 7 hours and is more than two years old. I can't remember the last time I had to stay six hours straight without having a plug nearby long enough to pump up some serious juice (and remember, charging is non-linear, the first 80% come up in a blink). Swapping batteries is a niche, yet it's the standard in PC land. I'd rather have a couple more hours in a thinner enclosure than twice but at the cost of lugging around a bulkier design, along with the second battery. Plus recharging two batteries is a chore.
The battery analogy is flawed.
[1]especially time machine and aperture but a lot of others as well