Agreed. I think OO is the better, more powerful application, but that horizontal view in Tree is so useful. I think, in part, because you can see more subtopics (displayed to the right vs below), which can reveal patterns in the text.
I had to submit my analysis of several training modules at work, and using Tree made things clearer to everyone at the meeting. They could see the current pattern (and errors) as well as quickly comprehend my proposal for a new structure. One guy immediately bought Tree after the meeting.
Column titles are a nice touch too, as is focusing by opening part of the outline in a new tab. It looks like focus of some kind is coming in v5. I'm on pins and needles for it to come out of beta!