It's not going to happen because it screws up links to thread pages. For example, let's say a thread has 300 posts and you have your preference set to 100 posts per page, but the default setting is 10 posts per page. Page 3 of the thread for you would contain posts 201-300, but if you ever copy and pasted the URL to that page 3 in an email or on another website and somebody clicked on it, it would show only show posts 21 - 30 for them because the default setting is 10 posts per page for an unregistered user or a member with the default setting, So page 3 for them would be drastically different than your page 3. That means they wouldn't be seeing the page of posts you were trying to reference at all.