More productive?
Perhaps the "more productive worker" got the job because they told their employer (resume, interview) that they were very 'efficient and focused'. Either way, I don't think being more productive ever gives you the right to 'goof off'.
If you had to go under the knife, would you be happy if your surgeon paused to check his twitter updates mid way through just because he's a "more productive worker"? (yes I know it's an extreme example)