In and of itself, a specially-designed festival space in the center of the park theoretically makes a lot of sense even if it had an expensive restaurant on top. I don't know whether the proposed building used space efficiently, but it reminded me of the redevelopment of an area in my home city of Melbourne, Australia called Federation Square. That involved a mix of restaurants, a gallery, and museum and was controversial as a lot of people found the architecture ugly & criticised all the space given to expensive restaurants, but it ultimately worked in large part because they built a lot of civic space into it and it became a place where people would sit and read, gather to watch sporting events on a big screen, etc.
At least from what the art suggested, they seemed to want to hold events under and around the festival center, with that design essentially creating an open-air ground level that could be used for a range of functions as well as the two upper levels. At least in theory, it's not quite the madness a lot of people make it out to be. I also didn't think it looked as bad as many on here seem to.
As for the Communicore buildings, I personally think they should have either knocked neither or both of them down. Leaving one is strange and ruins the symmetry of the center. Repurposing both would also obviously have been a good option. The way people talk about those buildings as great works of architecture, though, puts Zach's hyperbole to shame.