- Consistency: there's no FAQ namespace, though API15, API16 etc. are admittedly useful. However, tutorials do not have a significantly 'special' structure that would warrant a namespace.
- Flexibility: categories are easier to add and remove, not to mention the fact that categories can have subcategories and are easier to view an index of.
- Extensions: index articles such as the Management series use the DPL extension, which selects articles based on their categories and not their namespace. For example, http://docs.joomla.org/Tutorial:Override_JFormFields is not shown in the list.
- Use: http://docs.joomla.org/Category:Tutorials shows that either documentors are ignorant of the namespace, or they just like categories more, as the majority of the tutorials does not use the namespace. The list of Tutorial: articles can be found on http://docs.joomla.org/index.php?title=Special%3APrefixIndex&from=Tutorial%3A&namespace=0
I wouldn't mind doing most of the work of moving, but I'd like to get community consensus first.
By the way, shouldn't there be a central JDOC discussion page on the wiki for such proposals? On projects like Wikipedia, it seems that keeping the discussion right next to the discussed content works better than having a separate resource.