This patch for Yahoo Messenger 8.x & 9.x are used in order to run YM for more than one account. You can run 2 Yahoo Messenger in the same time. See the picture below.
Even so, I still prefer to use Digsby or Trillian Astra. If you run Yahoo Messenger, it will only be able to chat on the Yahoo network. If you run G-Talk, it will only be able to chat on the G-Talk network. So, by using Digsby or Trillian you can chat on Yahoo network….. G-Talk network….. with running one software.