One thing that's not mentioned is that pickled data is effectively fossilized once you've pickled it. If you want to change the layout of a class and have objects unpickle correctly, it can be an ordeal, as objects are unpickled by their class name, and you need both the original class and the new class to correctly unpickle and migrate.

If you instead selectively pick what you want to serialize about your data and keep the representations separate, you can change the internal model easily without having a huge impact on the serialized model.