A duplicate question, at times, can have a more correct title of the question made earlier.
In addition, a duplicate question, can increase the reputation of the person who gave the correct answer to the "master" question, because in a duplicate question, surely there will be a comment with a link to the "master" question.
Moreover, most of the links generated by a duplicate questions are internal to SO, this also increases the use of SO itself. So for me sometimes duplicate questions are useful, and sometimes i can learn new things.
As example of my first point, in the ios environment two question could be:
Master: "put pins in mapview"
Duplicate: "How to add MKPinAnnotation in MKMapView"
The duplicate title it's obviously better than the first question title, and maybe nobody ever will edit the title of the master question.
This is not really a question, I just wanted to know if anyone thinks differently :)