Perhaps you should consider using the Search function conveniently located at the top of the page.
A quick search of "Duke Bindings" will yield numerous threads on this apparatus:http://forums.epicski.com/showthread.php?t=51025http://forums.epicski.com/showthread.php?t=61059http://forums.epicski.com/showthread.php?t=61707http://forums.epicski.com/showthread.php?t=59187
Also, if you hit up TGR they have a TON of threads dedicated to the pros and cons of The Duke.
But the short, non-technical answer to your question (though "Why?" is pretty ambiguous, but assuming you mean "Why is the Duke better than the Freeride for going down more than going up?") is that the Duke is a lot more burly than the Freeride in that it weighs considerably more. It is also noticably more low profile (the stack height is more akin to an alpine binding than an AT binding). It has higher DIN settings.
If your question is "Why would you suggest that I buy alpine bindings over the Duke or Freeride?" the answer to that is simple. If you are mostly concerned with going down the mountain then there is no reason for you to spend $450 on either The Duke or The Freeride when you can buy a sweet alpine binding for $150. If you are interested in getting into hiking for turns, it's usually best to start with short in-bounds jaunts at your local resort to see if you really enjoy exerting maxiumum effort for often short amounts of extra vertical.
Just trying to save you money, my friend, if in fact you are more concerned with riding down the hill than hiking up it!