I realize that there is no need to provide non-retina images if you do an iPhone app for iOS 7.0 and above. Last non-retina device was iPhone 3Gs and does not support iOS7.
If you run your iPhone app on a iPad, it should use compatibility mode with retina assets.
Note: Xcode will not let you specify a non-retina image for LaunchImages in that case anyway.
You can refer to the useful table of devices: http://iossupportmatrix.com/