well, because of the stage at the east end the Square already has a front and a back...
Not necessarily. The only reason this has become the "back" of the square is because Victoria St. isn't so busy. There was space next to the stage and stairs to sit down on which would have been used had Victoria been a higher pedestrian traffic street.
Also, the green wall isn't really a green wall at all. A green wall has vegetation growing on a vertical platform. This will have plants growing in front of it, from the planters placed at the bottom.
In total fairness, the shed does look well designed -- this photo doesn't do it any justice -- but its location is totally inappropriate and not in harmony with the stage and the design of the square.
My preferred approach to this end of the square would have been to build on the idea of a sort of amphitheater facing the Rogers building. Double height steps built up to the level of the stage would form part of this amphitheatre. Rogers in turn would redevelop the torch podium as a stage for performances and live shows. People could then sit on the steps watching the show. People would sit along the stage and others would watch from the square itself. An opportunity lost I fear.
Edit: Here's what was here before:
The intention here was to create a gradual incline from Yonge to Victoria so that there would be no front or back of the square. This side provided stairs, another "entrance". The new wall is just that, a wall that blocks sightlines. I would have liked to see a continuation of these stairs to the height of the stage. This Google Streetview picture shows how the architects' intention was ignored and this space used for storage. It started with a place to put tables, chairs and barricades. Then the shed came in and it went downhill from there. Now the permanent shed, while more attractive, ruins the whole flow of the square.