The point being that if you have a stream of data that is the same for a large number of recipients, you can send the data down the tubes once, and it's then shared by recipients.
Although doesn't solve the general case (people wanting to watch the same thing, but out of order).
In ISP caches more or less solve that problem, as Netflix and others use, while allowing some out of sync views. At this point multicast just isnt going to make it.