if a spammer post in those threads and they get locked, the spammer get their reward from the signature campaign. If the thread gets trashed, he looses all the time and effort to post and eventually will give up.
exactly for this reason i tend to use "trashcan" instead of locking the thread.
This takes away the joy of those unnecessary threads that don't generate any added value for anyone
But there are no real guidelines for this.
i'm only responsible for the german part of the site and fortunately these spamthreads are very limited there.