Air conditioning is not as common in New England (where the author is located) as it is in the rest of the country. I would say about a third of projects I worked on in the Northeast did not have air conditioning, and I was working on pretty expensive houses.
Insulation/air-sealing can get you pretty comfortable in New Hampshire.
Yes, first time I visited somebody who constructed a well-insulated, air-sealed house with high quality windows, I noticed the owner routinely slid the window open partially in the winter to allow excess heat to escape temporarily. I think this is common in well-constructed New England homes.
There's a once-a-summer, or really more of a once-every-few-summers level heat wave rolling through the northeast right now...not something I'd necessarily take into account when buying a house. I grew up in very old unairconditioned house in inland New England, where I don't believe I knew anybody in the 90s/early 2000s who had central a/c, even in a fairly affluent area. From what I've read the models don't expect the region to face major summer heat effects from climate change.
When it's in the high 50s/low 60s evey night, and you have some thoughtful design so the house can be ventilated well with just the windows, the cool night air sticks around well into the next day, a cold shower or a little portable a/c unit can get you through the handful of hot nights each summer.
I live in NYC now, where a/c is necessary most nights in July and August, and find it kind of funny to see New Yorkers buying summer houses in my hometown and immediately spending thousands retrofitting central air. For me one of the great pleasures of going home to visit my family is sleeping with the windows open to the chilly night air!
Insulation/air-sealing can get you pretty comfortable in New Hampshire.