Well you could either initialize the xib using a view controller and use viewController.view. or do it the way you did it. Only making a UIView subclass as the controller for UIView is a bad idea.
If you don't have any outlets from your custom view then you can directly use a UIViewController class to initialize it.
Update: In your case:
UIViewController *genericViewCon = [[UIViewController alloc] initWithNibName:@"CustomView"];
//Assuming you have a reference for the activity indicator in your custom view class
CustomView *myView = (CustomView *)genericViewCon.view;
[parentView addSubview:myView];
//And when necessary
[myView.activityIndicator startAnimating]; //or stop
Otherwise you have to make a custom UIViewController(to make it as the file's owner so that the outlets are properly wired up).
YourCustomController *yCustCon = [[YourCustomController alloc] initWithNibName:@"YourXibName"].
Wherever you want to add the view you can use.
[parentView addSubview:yCustCon.view];
However passing the another view controller(already being used for another view) as the owner while loading the xib is not a good idea as the view property of the controller will be changed and when you want to access the original view, you won't have a reference to it.
EDIT: You will face this problem if you have setup your new xib with file's owner as the same main UIViewController class and tied the view property to the new xib view.
i.e;
The below code will cause confusion later on, if you write it inside view did load of YourMainViewController. That is because self.view from this point on will refer to your customview
-(void)viewDidLoad:(){
UIView *childView= [[[NSBundle mainBundle] loadNibNamed:@"YourXibName" owner:self options:nil] objectAtIndex:0];
}