But today's batteries, such as the lithium-polymer cells in the MacBook Pro, charge quickly and can handle a much greater number of charge cycles. That lessens the need to swap out batteries -- Apple is counting on that to be true, claiming that the batteries will be good for at least five years, which is longer than vendors expect most people to keep their laptop.
The combination of modern battery technology such as lithium-polymer and the greater capacities allowed by sealing them in also reduces the need to swap out a battery in flight.
How to handle the legitimate issues that sealed-in batteries create
Still, the risk remains that a battery fails or you need it to run longer than it is capable of. A sealed-in battery leaves you stuck in both cases.
Or does it? You may not be able to swap out a dead battery for a charged one in flight, but you can still use an external battery to power the laptop. These external batteries connect to the laptop's power jack, and are roughly the same size as a spare internal battery.
Sanho, for example, sells such batteries for MacBooks; they also let you plug in USB devices, such as iPhones. They connect via a power cord, so you don't need to place the external battery right next to the laptop (which would be impossible on most airlines' tray tables). The lowest-capacity version costs $200, or $40 more than the equivalent Apple internal battery. Battery Geek offers a similar product that works on a variety of PC and Mac laptops.
When batteries fail or swell and need to be replaced, it can be trickier to deal with them. Typically, you send the laptop back to the manufacturer to have the battery replaced, often at no charge. (In the case of a swelled battery, which is often caused by water intrusion, the laptop may be damaged and need further repairs or complete replacement.) In a business environment, that means IT will need a few spares to loan out as the laptop with the failed battery is out for repair.
A better option may be to swap out the failed battery with a working one from another laptop, so your user isn't delayed much in working. Despite being sealed in, the batteries are often user-removable. For example, although Apple requires that battery replacement be conducted by Apple-certified technicians, replacing a fixed battery in a MacBook Pro or MacBook Air requires only a #000 Phillips screwdriver and a few minutes at best. Theoretically, you could send the spare laptop that now has the failed battery installed to Apple for repair, hoping Apple doesn't notice the swap, say you've voided your warranty, and thus not replace the battery. If you're friendly with an Apple tech, you might quietly inquire about how Apple would treat such warranty "flexibility."
Why I'm comfortable with sealed-in batteries
My own experience with laptops leaves me at ease with the idea of sealed-in batteries.
I bought a spare battery for my two-year-old 17-inch MacBook Pro a month after I got the laptop, as I was about to head across the United States. On transcontinental flights, it's fairly hard to drain a battery of that capacity. On my trip, I never did break out the spare, and to this day it sits unused on a shelf in my office.