alt Hacker NewsServices can have a 50% uptime (or a 50% downtime if you prefer) as long as it's the time when I need it to be up (or don't need it.)
Which is to say that the significance of downtime depends on the user. Talking about nines only makes sense internally when you are evaluating your infrastructure and operations. It doesn't tell you squat about impact to your customer.
Right. And there's nothing wrong with having regular scheduled downtime. That's obviously not appropriate for most massive scale global cloud services, but it's great for services that know their usage patterns well.