Robert Pasnau penned an open letter (pdf) to prospective PhD students on why the history of philosophy matters:
[M]any philosophers today are presentists they think that the only philosophy worth reading has been written in the last 100 years, if not the last 30 years. This attitude is hard to justify.
The historical record shows that philosophy unlike science and math does not develop in steady, linear fashion. Perhaps the very best historical era ever came at the very start, in Socrates, Plato, and Aristotle. If that was not it, then one has to wait some 1600 years, for the century from Aquinas to Oresme, (Who’s Oresme?, you may ask. Exactly.) or wait 2000 years, for Descartes through Kant. I’m leaving out important figures, of course, but also many quite fallow periods, even in modern times. Maybe subsequent generations will judge 2011 and environs as the highpoint up until now of the whole history of philosophy, but I wouldn’t bet on it. Every generation of philosophers has been equally prepossessed by its own ideas.
(Hat tip: Brian Leiter)