This is OK advice if all you want to do is finish a PhD as quickly as possible (ignore course work and any studying not directly related to your dissertation), but it's terrible advice if you want to (a) become a broadly smart individual or (b) do research later on anything else besides the tiny topic of your dissertation. Grad school is the only time in your life where you'll have the freedom to study all sorts of incredible stuff. If you leave academics, you won't have access to the resources, and if you stay in academics, you'll be constantly scrambling to crank out publications as a post-doc or professors.
> In the interest of personal disclosure, I suffered from the "want to learn everything" bug when I got to Ph.D. school. I took classes all over campus for my first two years: Arabic, linguistics, economics, physics, math and even philosophy. In computer science, I took lots of classes in areas that had nothing to do with my research. The price of all this "enlightenment" was an extra year on my Ph.D.
Only a year? Sounds cheap to me! When else in your life are you going to have the opportunity for this kind of enrichment?
Yes, taking linguistics classes is probably not going to help your future physics research. (Though it can still be worthwhile). But taking chemistry, math, and physics outside of your particular niche is incredibly helpful for teaching you different ideas that you can apply later.
Also:
>Einstein's Ph.D. dissertation was a principled calculation meant to estimate Avogadro's number. He got it wrong. By a factor of 3.
Yea, well, for their PhDs, Bekenstein's discovered black hole thermodynamics, Feynman came up with the path integral, and Hawking proved the singularity theorem. Yes, you can go on to be successful after a modest thesis. But you can also kick ass young. (Also, finding a method to calculate a theretofore empirically-measured universal constant based on elementary principles--and only being wrong by a factor of 3--is pretty incredible!)
Frankly, I just don't know very many grad students who come close to "shooting too high". I'm sure this professor has met a few, but is his concern really that they are shooting themselves in the foot, or is it that grad students who take chances for big breakthroughs (and hence, usually fail) are not very useful for advancing the career of professors?