that's true. but i am not sure how good of an idea it is for 500 pages of a thread to get 500 new URLs if I made a small change to the title of that thread. that's 500 pages that google/etc would have to reindex and remove the old 500 urls from their index. wordpress is not a forum software but they decided for some reason that your blog post URL would not change (unless you want to) if you made a change to the title of a blog post.
though i guess the behavior is same as other major forum software like IPB or vB? i haven't used either in a while and I do not remember if they allow a slug option that is maintained even if you change the title of the thread?