The problem with deleting a thread is that often - hopefully - people have posted in your thread. So if it's deleted, their post count suffers, which matters to a lot of people, and whatever they contributed to the thread is lost. And to expect a mod to go through a thread and delete only certain posts is often asking a bit too much as it can be quite tedious.
There is also the issue of someone stealing someone else's account and asking for their threads to be deleted just to cause chaos. That's why most of the time a thread will be moved into the archive section instead of just being deleted.
I personally have no problem with someone sending me links to the posts they want deleted assuming they don't go overboard. And I normally delete double posts whenever I find them whether people ask for it or not.
If you have any other questions, just ask. Either here or in a different thread.
JB