You generally make class protocols weak to avoid the risk of a “strong reference cycle” (formerly known as a “retain cycle”). (Note, we now do that by adding the AnyObject protocol to a protocol’s inheritance list; see Class-Only Protocols; we do not use the class keyword anymore.) Failure to make the delegate weak does not mean that you inherently have a strong reference cycle, but merely that you could have one.
With struct types, though, the strong reference cycle risk is greatly diminished because struct types are not “reference” types, so it is harder to create strong reference cycle. But if the delegate object is a class object, then you might want to make the protocol a class protocol and make it weak.
In my opinion, making class delegates weak is only partially to alleviate the risk of a strong reference cycle. It also is a question of ownership. Most delegate protocols are situations where the object in question has no business claiming ownership over the delegate, but merely where the object in question is providing the ability to inform the delegate of something (or request something of it). E.g., if you want a view controller to have some text field delegate methods, the text field has no right to make a claim of ownership over the view controller.