I'm not sure that it's so much what Slytherin contributes as how best to deal with the Slythery people in society. Best to put them in school to keep an eye on them, then to put them in their own House to keep them from bothering the others. Even if you would prefer to be rid of them entirely (as Rowling, at least, might be), that's not possible.
Update: This post has also been superseded - new comments belong in the latest thread.
The second thread has now also exceeded 500 comments, so after 42 chapters of MoR it's time for a new thread.
From the first thread: