Why is constituency needed, since dependency gets the job done more easily and economically?

As a proponent of construction grammar, I am perhaps the wrong person to answer this. But I can see at least two non-computational advantages of a constituency parsing:

1. It lets you directly encode generalizations about the individual components that make up a clause or sentence. Your constituency tree does not take advantage of it (perhaps being influenced by dependency approaches).

2. It does not require a commitment as to the head of every phrase. So let's say you change your mind about verbs heading the VP, you can do it without changing the parsing.

Of course, you can get the best of both worlds with something like Categorial Grammar or Unification Categorial Grammar.

But I could be wrong both about my answer or about the point of your question.