Choosing the wrong type of object to use in your application can have bad implications for your app on several levels. For instance, your app could suffer from unwanted side-effects when a reference type is modified in some unexpected place. Or you could end up with a lot of duplicated logic if you use a struct instead of a class in certain places. Your app could even suffer in terms of performance when you choose a slow reference type where a value type would have been a better choice. You should always evaluate what type of object is best-suited for your current use case to make sure your code strikes a balanced trade-off between maintainability and performance.
A great time to use a reference type is when you are subclassing a built-in class, such as UIViewController. In these cases, there is no point in fighting the system because that would definitely do more harm than good. Another time to use a reference type is when you...