The reason why is that not all system classes have been compiled with ARC.
You may implement a dealloc method if you need to manage resources other than releasing instance variables. You do not have to (indeed you cannot) release instance variables, but you may need to invoke [systemClassInstance setDelegate:nil] on system classes and other code that isn’t compiled using ARC.
See this page on developer.apple.com: Transitioning to ARC Release Notes