Well it also opens Pandora's Box of Devs offering free apps then using their own IAP system to avoid paying their fair share to Apple.
That is assuming customers trust the app with their CC numbers.
I don't have a problem with buying ebooks in the Kindle app (and audiobooks thru Audible), because Amazon has my CC information. They have had it for a long time. And I am not going to give my CC to a third-party IAP just for in-app purchases. I will delete any app that won't allow me to purchase in-app through Apple.