I kind of agree with the author, although I see your points. If you are talking about getting a broader understanding of your subject then that's pretty much part of the phd and nobody will argue against you for studying things that you think _may_ be relevant.
On the other hand a PhD is not the place to study Arabic or Sociology if you want to be a high energy physics: you can do that, but in your free time, not in the University paid time. For a long time I thought it would be great to be a student all my life and just go to lectures and learn. But nowadays I know that if I want to learn I can pick up a book and learn. That's how a lot of people do: undergraduate is there to give you the tools but also to teach you the method.
Anyone worth their salt with a phd can study another topic by themselves: you don't need to be lectured and have tutorial anymore.