Check out verify.ly to get some surprising info on what iOS apps are doing behind the scenes. There are plenty of apps that have the ability read your full contact data or your calendar without ever asking you. Skitch, Amazon, and Skype do one or more of those. I have deleted them and gone back to the web app for Amazon. FaceTime replaced Skype, and I'm stuck with Markup in Photos as a poor replacement for Skitch.

Of course there are plenty of safe apps that do not abuse APIs - for example, the Deliveries app by JuneCloud.