One does not need to use the signup in the app at all. I both pay for Spotify, and use the iOS app, but have never paid in the app.

I trusted Spotify enough to sign up directly via their website, but for many apps I do not, and prefer to pay via the app store as I trust Apple.

In my personal opinion, this experience is pro-consumer. I can make an informed choice. Hence I don't take any issue with Apple requiring developers to a) pay a cut for the privilege of using their store, and b) impose publishing requirements to maintain the standard of apps.