Graham Priest (CUNY, Melbourne) presents a brief defense of the history of philosophy over at the OUP blog.
One of my friends said that he regards the history of philosophy as rather like a text book of chess openings. Just as it is part of being a good chess player to know the openings, it is part of being a good philosopher to know standard views and arguments, so that they can pick them up and run with them.
There is a lot of truth in this analogy, but it sells the history of philosophy short as well. Chess is pursued within a fixed and determinate set of rules. These cannot be changed. But part of good philosophy (like good art) involves breaking the rules. Past philosophers may have played by various sets of rule; but sometimes we can see their projects and ideas can fruitfully (perhaps more fruitfully) be articulated in different frameworks—perhaps frameworks of which they could have had no idea—and so which can plumb their ideas to depths of which they were not aware.