Well, presumably historians do specialize. In the revised world where history of philosophy ended up in the history department, there would be historians specializing in the history of philosophy. For that matter, I'm sure such people exist already.

The real question is which option provides more synergy:

1. learning about the motivations for Aristotle's theory of Entelechy, together with a study of the culture of Greece in the 4th century BC (the historical option), or

2. learning about the motivations for Aristotle's theory of Entelechy, together with a modern understanding of causality or whatever (the philosophical option).