Via Brian Leiter. The argument proceeds from the premise that to understand any philosopher, you have to understand the philosophers who influenced him/her. This pushes you back down the history of philosophy until there's nothing you can write a good dissertation on after Aristotle. (People in well-funded doctoral programs might be able to make it up to Epicurus.)
Obviously things aren't that bad. The big point to be made here is that it's impossible to produce work that comes from a perfect knowledge of everything. You pick the things you're going to be really good on, and you're going to be mediocre at some other stuff. Some of us are going to know Montaigne and we'll be able to understand what Nietzsche is saying about him, and what that means about Nietzsche. Others are going to know metaethics and we'll be able to characterize Nietzsche's metaethical position in more precise terms. Maybe imperfect knowledge of the areas we're not expert in will impair our efforts. That's why we talk to each other.
Also: I admire philosophers who do interdisciplinary work that engages with the humanities and social sciences. There's lots of bad philosophy out there, and the world seriously needs philosophers who have the intellectual ability, patience, and academic social skills to help people there see that their (often very worthwhile) projects shouldn't be shaped by bad theories.