pointless question, I just wondered if any topics get deleted by admins or are they only shut?
do mods ever delete topics too?

Generally threads are not deleted unless they long are simply strings of cursing or other behavior specifically not allowed.

I guess if a thread only contains swearing and insulting others it gets deleted, otherwise it gets closed.

Even in that case they are often closed instead of deleted I think.

Duplicate threads also get deleted, naturally. I don't mean copycat threads, but real duplicates.

Such as Person A posts "I love pie!" and Person B posts "Pie is cool!"?

Such as when you accidentally post the same thread twice in a row.

Wouldn't merging the threads be better? (Like in Fawriel's triple birthday theads).

I know spam topics posted by bots get deleted, as well as some illegal Jazz download (warez, JJ3, etc...) providing or requesting topics.

I think threads that may incite:
racism
anger
hatred against a particuler person/group of people unnesesarily
spamming

Should be deleted.

And another question since I won't bother making another topic..... how come the word CRAP is allowed? [:

It's too informal and perfectly non-vulgar. ;P

And I can say you are full of crap and you smell like crap.
Get my point? don't take those as personal attacks, it's just examples...

Derby: Content removal. The removed content could be inferred from the remaining context of the post. The severity of the content changes based upon the words' connotations.