One good reply is that studying the history of economic thought can save   
individuals the time and expense of reinventing the wheel.  E.g. The   
debates about contestability (i.e. the power of potential entry to   
discipline even firms which apparently had substantial monopoly power) from   
the 1980s are echoes of the arguments of John Bates Clark, Jeremiah Jenks   
(See my article on Jenks in the Journal of the History of Economic Thought   
(March 04) for a discussion of other parallels to later economic thought),   
and others in the 1890s and early 1900s.  
  
John Howard Brown