Just out of curiosity - it seems that keeping around questions which are closed as duplicate makes sorting through older questions more cumbersome. Any reason they are kept around?
Sometimes people ask the same question in very different ways. Duplicates act as signposts. Often when I'm searching a site for a question, the top result isn't the question itself but a duplicate -- because I and that asker happened to use the same language. So the duplicate leads me to the original, but I might not have found the original at all otherwise. (For example, I found this via a duplicate, not the original.)
Duplicates that aren't likely to serve that purpose -- ones that are cryptic, for example -- can be nominated for deletion. But in general, it's better to err on the side of keeping them in my experience.