There is no impact at all. Apple engineers thinks that its not required to hide, or may be Plus phones will get landscape slide over in later iOS versions :)
From the documentation:
To opt out of being eligible to participate in Slide Over and Split View, add the UIRequiresFullScreen key to your Xcode project’s Info.plist file and apply the Boolean value YES.
Normally, you would need to support both portrait and landscape orientation in your app.If your app is landscape only or portrait only, then the app validation fails with:
warning: All interface orientations must be supported unless the app requires full screen.
And as KingofBliss noted, setting this to YES will also stop your app from running in a split view.
It is a useful setting for games that run full screen in landscape only.
