Example of why you shouldn't blindly throw away type information from the compiler when casting to a protocol

gistfile1.m

- (void)someCallbackWithAViewController:(UIViewController *)viewController
{
	if ([viewController conformsToProtocol:@protocol(CrazyViewController)])
	{
		UIViewController <CrazyViewController> *crazyVC = (UIViewController <CrazyViewController> *)viewController;
		if (crazyVC.presentedViewController == nil)
		{
			[crazyVC showCrazyModalView];
		}
		else
		{
			[crazyVC startCrazyViewHideTransition];
		}
	}
	else
	{
		// do something different
	}
}

If you had cast crazyVC to (id <CrazyViewController>), you would get a compiler error on line 6.