As of yesterday I had an app using mostly static sizes to fit an iphone 5 screen (320 points width). It was working fine in iphone 6 as well thanks to the system scaling up automatically.
Then I decided to add a retina hd launch image and everything became a lot smaller on iphone 6. So I decided to modify my whole application to use dynamic sizes and fonts so that it would fit the two iphone types the same way. And now it is working quite good.
But after these few hours of extra work, I keep asking myself the same question... what was the point? Why would you want to use dynamic sizes that fit both iphones when you can make it work for iphone 5 and let the system scale up automatically?