It is useful to be able to capture serialized structs without necessarily pushing them over a pipe right away.

For a concrete example, mash wm stores generic window properties as array<uint8>. Currently there are custom TypeConverters to go between real types and vector<uint8_t>, which is unfortunate. Ideally you could just say something like:

  mojom::RectPtr r = ...;
  mojo::Array<uint8_t> bytes;
  r->Serialize(&bytes);

and

  mojo::Array<uint8_t> bytes = ...;
  mojom::RectPtr r = mojom::Rect::Deserialize(bytes);

We could trivially generate implementations of Serialize and Deserialize for all mojom structs using the generated Serialize_ and Deserialize_ functions.

It would be worth considering this (though IMHO less obviously a good idea) for interface messages too since they are in practice just structs themselves.