The reason they leave them locked most of the time is so the users can have time to complain and justify why they think they think it shouldn't have been locked.
But topics that ask for illegal content and such should definatley get an instant delete.
We should do something like the spring cleanup next year. Except this year we could move old posts into an archive forum so people wouldn't get pissed about losing posts. I dunno... maybe.
Or say, for example, a useful tutorial that has gone off-topic. There's no point in keeping a spammed thread open. Someone with a question can just PM the author. However, locking the thread lets people see it in the future.