Pfft. Turn on your brain man!
<p>
Broadcast NEVER cross subnets. Multicasts don't cross subnets if you don't have a multicast router (which is common, I agree) or you set your TTL to 1. The most common configuration today is a switched network that sniffs the IGMP membership messages and will only flood multicasts to interested hosts, thus reducing any potential negitave impact on the network. Sure, the multicasts can't span subnets on most networks today, but if you would bother reading what I frigging wrote you'd see that I was saying that they were simply a smarter then broadcasts, even in the TTL=1 case.. Even in the most old & crappy network, the multicast traffic will still reach all the hosts it has to (in such a network it will be functionally the same as broadcasts).
<p>The IETF generally considers the use of broadcast to be depreciated for any new protocol (look at OSPF, broadcast would work fine, but why send the packets to unintrested parties).<p>Finally, most affordable switches don't give you the tools you need to block those broadcasts, you simply can't filter out all broadcasts as you need ARP on most networks. <p> If you want to be ignorant, thats fine, but please don't influence protocol design in areas you obviously don't understand.