It would be very useful for (pt.)wikibooks if the sort key of main namespace could be the subpage name instead of the page title (unless you overrode it).
This would save work when adding the chapters of a boot to the categories wich are used primarilly for grouping those chapters, since they are of the form "Name of book/Chapter" and so are by default under "N" (the first character of book's name)
At Wikipedia, there is no need to set the index or to use DEFAULTSORT in EVERY article page, because the article's title can usually be used for sorting (since the use of subpage isn't as necessary as it is for Wikibooks). But at wikibooks, almost every page currently needs to have an index...
The behaviour could be set according to the needs of each Wiki project (bt means of variables in Manual:LocalSettings.php or anything else)