The reason small sites than the leading social networks need to do this is because people are already chatting with their friends on the other networks. So by integrating all of these chat services together you can sign into Justin.tv chat with your Twitter, Myspace or Facebook account. Without having to create a Justin.tv ID.
This kind of integrated chat will definitely grow to be the new thing on the Internet. This will alllow smaller sites to allow people to sign in with any of their Social account, without having to always create a new account.
Justin.tv also said that you will now be able to import all your avatar's and enjoy the chat experience.