It's too early to be making such judgments regarding the availability of 'public' space on Toronto's waterfront. Sugar Beach and Sherborne Commons are two relatively small parks within the larger plan, which includes a substantial park in the West Don Lands, as well as Lake Ontario Park which will connect new developments in the Don Lands/Bayfront district to the existing 10KM-long Leslie Street Spit. Deriding WaterfrontTO for wasting 'valuable' public space before the project is complete does not do them justice.
As for Corus Quay and GBC as having taken up perceived public land or 'entertainment space', it's important to keep in mind how exactly these public spaces get funded - land has to be sold in order to receive the necessary funds to construct the parks (unless you think the city has the funds to build them), and frankly, these buildings could have been sold to much worse. Corus includes a restaurant and cafe, as well as features which allow for open air concerts during the summer fronting Sugar Beach. GBC isn't a self-enclosed institution either - they provide a variety of educational services to the public and will draw in non-area residents to use the public spaces.
It's naive to think the city is starved for public space - Toronto contains more green space than most cities, all it takes is looking around at what already exists in the immediate area - the Don Valley has a massive trail system which is begging for greater public attention, while Tommy Thompson Park is nearby, as is the Toronto Island.