The problem with using multicast for content delivery is more down to the subscriber end and how ISPs manage their networks. I worked on a (recent) project that uses multicast in the broadcast context at the mezzanine level. When you've got full control of the network, end to end, it works a lot better.
The subscriber end...and the goddamn Internet backbone. You can't route multicast across the core, which is a huge impediment to its adoption.
There was a vicious circle where the core didn't do multicast so the big iron routers didn't put multicast support in hardware, which made it impossible to support multicast on the core...
