As of right now, the only ones with complete viewership data is Twitch. Game Developers can access a limited set of this data through the VHS system, but that’s limited to channels streaming their games, and requires the viewer to opt-in by connecting their account to the game.
By allowing 3rd parties access to view channel points of users, that would make it possible for the broadcaster to grant that scope and let their viewers be tracked with absolutely no opt-in, or opt-out, on the viewers side other than to watch Twitch logged out.
As you scale up, to a multi-channel 3rd party service, you would now potentially be able to view how a user moves between channels, and even potentially stalk them using this data. Think of some of the big 3rd party companies that offer overlays, extensions, point systems and stuff already on Twitch, they would have access to such a huge amount of viewership data that it would go beyond simple points systems usage.
Access to viewership data through the points system has incredible value for analytics/marketing uses, and there will be some 3rd parties out there who request that scope to access the data from a broadcaster not because they care about the points system but because they want access to the data for marketing use, and the viewer has absolutely no say in that, as if they’re logged in and watching a channel Twitch’s points systems track them.