Is Google Authenticator really useful on blockchain.info?
Two-factor authentication is generally useful in that it adds a different layer of security to your account. In addition to your password (which is something you know) two-factor authentication adds a key generated from a specific device (which is something you have.)

In my experience one can never be too cautious about their financies, especially when they are being hosted on a machine connected to the internet. If it were just the security vulnerabilities you knew you knew about (e.g., the strength of your passphrase) that would be one thing. Usually it is the security issues you do not know about that justify extra layers (like two-factor authentication).