I also own an original MacBook Air, the first Mac with a sealed-in battery. After 18 months of daily use and 325 cycles, the battery capacity is down to 94 percent, but it still lasts nearly three hours on a full charge. I haven't needed more time than that, even on long flights. If I did, I'd get an external battery and be done with it.
My Dell D800 is another large laptop that has the original battery -- and no spare. The lack of a spare for that laptop has never been an issue, either, and I use laptops like they owe me money.
I also know that if the sealed-in batteries are not up to snuff, the manufacturer is on the hook for battery replacements that are costlier than just shipping a new battery to the user. So they have a strong incentive to make sure the technology works.
Given today's technology, it should make no difference whether the battery is user-replaceable. The benefits of current battery technology -- extended runtime, weight reduction, and better overall design -- simply outweigh the occasional need for a spare battery. And the few who do need spare batteries can get an external one.
If the foray into using sealed-in batteries by Apple and Dell are successful, don't be surprised to see more manufacturers join the club.