For the vast majority of users, isn't it 1, by a _lot_? OS-integrated password managers that generate unique secure passwords are slowly helping here, but I suspect most people have approximately the same password for their email, bank, and Netflix.

I don't think that it's this obvious. The number of passwords stolen from the server goes in the hundreds of millions annually on average (well, at least the cases made public). While I don't have any numbers and reusing a password is clearly unwise, I don't think that there are that many Netflix employees getting a kick out of posting on Twitter under their customer's name.

It hinders keyloggers, protects against password stuffing, and if enough Twitter accounts are hacked they're just sold off to the highest bidder.