v4 will not support device specific assets at all. Adding two more image suffixes for the two new screen sizes is exactly what we were trying to avoid. It will still be possible to load them, but you'll be on your own for it. Requiring people to make 4-7 versions of every image is insane. It's an absolute waste of disk space for a problem that is better solved by cleaning up the image loading code.
You should care since a number of your problems are caused by floating point precision. If you treat the screens natively, then you get integers and everything can be computed exactly.
Like I said, you can treat them the same, and you are. It clearly mostly works mostly fine for now, but it's still not a good idea to make this be the default behavior since that isn't how iOS works. It also is a bad idea to hard code this assumption into Cocos2D. Assumptions like that in the past are what have caused almost all of the growing pains that Cocos2D has had lately. You may be confident enough to bet $100 that Apple won't change the aspect ratio again, but I won't bet Cocos2D on it.
Ignoring the display zoom feature for the moment, the iPhone 5 and 6 displays are the same DPI and they use the same assets. A UI element or icon on an iPhone 5 display is exactly the same size in both pixels and centimeters. For the iPhone 6+, Apple ensures that UI elements render at the same physical size with the goofy 2208 -> 1920 scaling even though it's only a difference of ~15%. Apple has clearly gone to a lot of trouble to make it work this way instead of just scaling up what you see on an iPhone 5. This is also the way Cocos2D is set up to work by default, and that is not going to change.
Lastly, when you enable the (non-standard) display zoom feature, it renders everything at iPhone 5 resolution and then upscales it. Most people don't turn that on because it makes everything blurry.