Deleting one's own threads means that one can delete one's own threads, not those of other people. I don't understand how being able to delete your own threads would fix the problem of "dumb topics". Unless you consider your own topic "dumb" because of the replies... but that still isn't right to delete it. People would be deleting topics if they don't like the opinions of people replying or if they are losing an argument, amongst other things.

A real life example: Publishers/authors can stop the sale/propogation of their works. But they can't get rid of public responses by others. If you think you said something embarassing or "dumb" in your post, you can edit it out. But you can't destroy the responses of other people.

If the thread was an accident, or you change your mind soon after posting (not because of the replies), an administrator will be happy to delete it for you. But if you want the thread deleted because you don't like the replies, you will rarely get your wish. It is at an administrator's discretion what to do with a topic. If it's just turning into a flamewar, they will close it. If it's turning into spam, they might delete it. They'll consider input from users but it can't just be up to the author.

(Note: This is my opinion, not necessarily the official position of the board administration. It would be good if other administrators replied to this.)