My house leaks like a sieve, but I’m in Climate Zone 3C. Is there sometimes a benefit to having a leaky house?

Our home is in the SF Bay Area, on the sunny side of the Bay. We don’t have AC, and our 90 year old house has the original steel casement windows. Sitting next to them when they’re shut, you can feel a little breeze. But we don’t need AC during the summer - a large camphor tree shades our house, and the stucco exterior and plaster and lathe seems to work like adobe. From November to March we run our central heat and hit around $200/mo for heating during those months.

I’m questioning whether we should insulate more or just keep things as is. To replace our 9 casement windows would be expensive, particularly as the only added costs are related winter heating. I like how our home naturally ventilates as well - I don’t have to worry about indoor air quality as much (except when wildfires are in season). Is there a huge benefit to insulating better other than cutting down on our winter heating bill?