How would you use broadcast or multicast to distribute an OS? Call me ignorant, but how would you do that in practice?
I used to work in a computer lab at the university I went to. We used Ghost to do exactly that. All the computers in the lab (a few hundred PCs) were booted up off a special floppy (or later, cd) that started up Ghost in listen mode. Then, the central server used multicast to send the OS image to all the clients at once. It took less than an hour, and that was with 100Mb/s Ethernet.