Indeed, as long as you only log in from trusted devices, you can just as well use private key-based authentication.
This kind of two-factor authentication solution is good in case you have to log in from non-trusted computers to your private server sometimes. It prevents someone running a keylogger from being able to log in. And you'd never copy your private key in that case as it could be easily stolen.
OPIE one-time passwords are similar and have been used for a long time. I don't know the advantages and disadvantages of the Google approach (except that you have to trust Google).
This kind of two-factor authentication solution is good in case you have to log in from non-trusted computers to your private server sometimes. It prevents someone running a keylogger from being able to log in. And you'd never copy your private key in that case as it could be easily stolen.
OPIE one-time passwords are similar and have been used for a long time. I don't know the advantages and disadvantages of the Google approach (except that you have to trust Google).