The problem is that some ISP will mistake your BitTorrent client downloading Sintel for a botnet and have the random ports for 30 of the 32 peers you can find blocked. Then, a new botnet will use a new zero day attack and masquerade as something users are guaranteed, such as iTunes radio. As soon as that is patched, some new game will use a different port that's blocked and tick off another customer. It will be a vicious cycle, because ISPs can't dictate what software you are allowed to use. Botnets and worms, however, have no legal purposes, and no sane law-abiding person would want to have their machine be a zombie on the internet.