I have noticed it is quite the pain in the butt to click on a [+] expecting a sub-category to be there, but frequently being greeted by a 'no subcategories' message.
This is fine, and definitely should be available for those who like it. It is a bit of a click-waster though. I would imagine a smarter GUI would not display a [+] if no sub-categories existed. Instead, it would render as a bullet-point or a [-].
At least something should indicate to the user that clicking on this particular option is a waste of their time.
This, however, requires additional lookups to the database. I suppose this should be done after the tree has initially been drawn or else the tree might take a while to appear. For performance reasons this should be optional and off by default. I don't believe this should be terribly expensive, especially if implemented well on the page.