It represented one of Apple’s biggest recurring flaws: letting cool come before functional.2With Ive’s new role leading UI design, I was afraid that we were in for a long series of such failures. And with iOS 7 being unveiled so publicly and confidently, I really didn’t think any decisions as significant as the system font would change before release. - Marco Arment
I’m not sure cool is the right word. Function follows Form follows Concept is something that applies better to Apple's choices.
Best example: the original MacBook Air:
- Concept: We want the thinnest Mac ever
- Form: Beautiful thin Mac
- Function: slow, and terrible battery live.
But since Apple iterates on their concepts, and technology always catches up, the latest Airs are stil thin, beautiful and have an awesome battery life.
In their quest for simplicity, and especially putting content first and let the UI dissappear, they went with the thinnes font possible. It looks great, makes their concept work, but the result is/was a UI that wasn’t always as readable as it should be. Luckily they itterated on that during their beta. And they’ll probably improve upon it with iOS 8.
(Title credits go to @Brentdax)