UI research is always interesting but it tends to be kinda useless. Depends too much on user group, other navigational elements present, content context and other variable.
I've been developing UIs for several decades and know that this is a pet peeve of users. Even though the tree control with checkboxes used on the advanced search page has some accessibility problems, it has a far greater usability benefit.
It w/b better assume admin users would have the same preferences for interacting with categories as public users accessing Advanced Search
PS This is very odd but the scrollbar isn't the full length of the text box in FF3.5. I can only view 3 lines at a time.