In addition to the privacy concerns others mentioned, let’s say the streamer uses the twitchnotify message to call new subs out, then he might still get the USERNOTICE for the same sub a minute later, so is he then supposed to call out the same sub a second time? You can’t just ignore the USERNOTICE either because it may have a custom message attached. It’s just impratical.
I’m glad that these systems are finally consolidated. The only thing Twitch needs to fix is that the Share-button shows up without having to reload the page, because a lot of people understandably don’t know about that and it’s a bit counter-intuitive.