Any plane ride lasting longer than a couple of hours requires a battery swap. I don't care how much they say battery life has imporved -- there's no substitute for having a second charged battery available.

There are plenty of other times when plugging in is inconvenient/impossible for long periods of time, especially when you consider charging takes an hour or two, and you might not be in one place for tht

To directly answer your question, I've probably *had* to swap batteries dozens of times, and done it voluntarily (where plugging in was possible but not convenient) hundreds of times.

The external battery packs don't help unless Apple licenses MagSafe, which they haven't. I heard there's a company out there recycling old Apple power supplies with MagSafe and putting external batteries on them, but I don't really trust that.