It is somewhat typical for certain activities to "feed into" multiple high-level goals or needs. Trees can't express that, so you get into activity categorization problem. Trivial example: are you cycling your way to work because it contributes to your health, is fun or because it enables you to do your job duties? Obviously it's all of that. None of that matters until you try to introduce some "time budget" for different spheres of your life, and with the tree concept you realize that the results don't make much sense.