the reason people want this is even as a modem user you are limited by a modem's upload speed of 31K on a 56K modem. so a modem user can get faster downloads from two sources at once. most cable modems have upload limits too.
cable modem users can suck the bandwidth from several thousand modem users at a time and get everything they ever wanted in 2 seconds.
thus cable modem users will prevent modem users from the benefit of this feature because all uploads are taken.
ok, so a modem user gets the file faster. now does he turn his client off cause he's happy? no, he's not happy cause he wants it all and wants it now and will never be happy. he will even go buy another 160GB hard drive if his gets full.
so he goes on to the next file and the next file and modem users get screwed.
does he share? not much! it might slow down his downloading a bit!
and developers are acting like this is something good.