Most people wouldn't be comfortable with a stranger looking over your shoulder while they logged in. This is the same thing, only you don't think about it.
These things are ALL rare, but why would you want to expose yourself to this?
SSL everywhere is also about improving security for those who don't realize that they might be engaging in behaviors that compromise their own security.
That is a completely different problem that using https won't solve. It's like building a ship with a hole at the bottom and having a high throughput water pump.
Also note that as tech-savvy people, we have more responsibility in ensuring our users are safe, even from themselves. Sure, the better thing to do would be to educate everyone so they don't reuse passwords. But it will take time, and using HTTPS in the meantime decreases the chance for them to be pwned.
SSL everywhere isn't yet practical only due to the expense. It's not that much more effort to secure a site but when you run 10 sites then you're spending $100 a year for those domains. The expense of an SSL certificate each on top of that makes it impractical for solo "webmasters" to secure all their sites. We all know why we should use HTTPS and we do it when it makes sense but it's just not practical 100% of the time yet. Like others have said, this will make more sense once the EFF initiative starts being adopted and getting a free certificate is as easy as apt-get secure-me-please.