Thanks for suggestion... I think it was offered some time ago; however we still have not implemented it (and I still have no closest plans for this) because we think, before deletion, topic must be checked in all aspects... and if it comes to an unexperienced user, like moderator or even admin, he can delete even useful content that way and then wonder why it was happened.
Additionally, when deleting a topic, a lot of resources are needed updating the topics count information for forums and updating messages/topics count information for users. Deleting large topics that way there is a big risk of the script expiration after X sedonds specially if your forums are running in safe mode.
So in the future I will think about it, but not this year probably ;-)