This is a critical characteristic of truly revolutionary developments- not merely augmenting or improving existing functionality, but rather making possible a new array of functionality that was previously unfeasible and paving the way for the inconceivable. Where the Web and the browser were the enabling technologies for the multitude of innovations that we have since experienced with the Internet, multicast will be the critical enabler for an unforeseen world of new functions and services towards which NextGenTV will evolve.
Maybe you're still not convinced about multicast. With all the other components well in place - cheap, intelligent end devices, ample access bandwidth, and willing participants already stepping into the ring (see AppleTV, GoogleTV), it is fair to ask if multicast is really necessary, especially in the on-demand world in which we now live. After all, video seems to sort of work just fine in the unicast-only Internet of today.
While it's true that there is a plethora of video on the Internet today, and the promise of a new wave arriving imminently, the numbers simply do not lie. Unicast delivery of high bandwidth multi-destination content is expensive, while multicast delivery is cheap. What unicast solutions may be able to provide is a certain niche of functionality - content with an audience that is large enough to afford the high cost of duplicate transmission, but small enough that the immutable laws of large numbers make it impossible for the network to deliver.
Such a niche world may indeed find modest success in adoption, but it will serve as only a diversion from what we think of today as television viewing. It could never fully overtake cable television and deliver all the revolutionary features and applications outlined here as NextGenTV. At best, a unicast-only solution could augment or improve existing functionality, but only multicast can deliver the unforeseen.
Where unicast may deliver "good enough, most of the time, for most existing content," it is critical to examine what happens during extraordinary times. After all, television has had its greatest impact during moments such as natural disasters, political speeches, military conflicts, assassinations and other times of great historical significance.