I also use nobita's messages per page addon. It's been a huge relief for me when editing very large threads cause I can set my preference to 100 posts per page without affecting everyone else. Should be core.
Won't allowing the users to change the per page counts cause links pointing to forum content to break? I thought there was an issue with the way the script is written that makes it impossible to change per page settings without breaking links.
Default option 20 better than others because we reading more messages per page and it's very easy to read more messages and it's pretty good for SEO. If option selected 10 for per page, it will be hard to read and follow thread.