The key here is the distinction between practical and impractical knowledge. The author seems to operate under the assumption that only practical knowledge is good. In her experience, she has been able to gain practical knowledge at least as efficiently outside of the graduate educational system. I agree with her point here, but I don't think you can discount the value of impractical knowledge - which I would define as knowledge that does not have any immediate real world application.
The value in impractical knowledge is twofold: 1) it can (and often is) able to called on later and 2) it can serve as an introduction to other practical knowledge. For instance, even the lawyer that the author has argued has wasted his/her education by transferring to a different career field may immeasurably benefit from some theoretical aspect of his graduate experience. More likely, an experience in graduate school may introduce a person to a subject area or conceptual idea that has a practical application they would never have been exposed to outside of that environment.
Some may argue that both of the above benefits can also be obtained through real world work experience, which leads to the question of knowledge for its own sake. I personally am a believer in learning for learnings sake, although I'm not sure I can articulate that argument in a comment length...