I got really excited over that because I thought that we now will see plenty of useful apps that will support this special landscape mode.
Now soon three years later, we have quite a few apps that supports it, but that's really not the problem. The problem are the apps that doesn't support landscape mode at all. Flipboard, Google+, Facebook, Instagram, Copied to name a few. There really are thousands of them.
Flipboard is an example that doesn't support Landscape mode
The problem is that when I am working on my iPhone 6s plus, writing in Ulysses, Drafts or Editorial (all of them supports iPhone plus landscape mode) for example, then I use it in landscape mode. And when I need to switch to another app for research or maybe replying to a comment or message. Maybe even by clicking on a notification at the top of the screen. Then if that app doesn't support landscape mode at all, then the app starts tilted and I am forced to change my phone from landscape mode to portrait mode. Really annoying. Especially if you use your iPhone for work as much as I do.
So I think that Apple has to force app developers to support landscape mode so that all apps can open in landscape instead of always in portrait.
And this will be even more important if the rumors are true that the iPhone 7 plus will have Apple's Smart Connector. Because then we will see a lot of accessories that will use the device in landscape mode. For example Apple's own Smart Keyboard. And as a developer, can you understand how annoying it would be if your app doesn't support landscape mode for all those users that using the iPhone 7 plus in landscape mode with the Smart Keyboard attached? I thought so.
Now go and add landscape mode for your app so that your are one step ahead of time for once. Then you will be better prepared for the future.