Take Gson, for instance. I expressed my frustration about this yesterday. Gson only works with fields. There is no way to make it work with setters/getters without writing custom adapters for just about everything. And having Gson work with the delegate field - without messing up the Json format - works fine with serialization (if you write a custom adapter for that), but fails during deserialization on generic delegates.
If the delegated property had had a backing field in the containing class, this scenario wouldn't be an issue.