>For broadcasts of live events (political speeches, Sports events, etc) there may be a small niche, but is that really worth the effort of implementing it across such a large infrastructure?
There actually seems to be a fairly large market for live-streamed video, like Twitch and Ustream, or YouTube's live option. However, I don't think any services like this actually try to multicast over the internet; it's unicast all the way.
So I guess the answer is 'no' - it's not really worth the effort, even when live video broadcast to tens of thousands of destinations is your principle business; possibly once it scales up to millions is where things start to look different, but at that volume you can invest in the infrastructure to avoid having to send it over the internet.
 Last year's edition of TCP/IP Illustrated still describes multicasting over the internet as "ongoing effort...for more than a decade", which seems to correlate with the general consensus I got from Google of "don't even try"