@silverpill regarding size, ActivityPub is such a verbose protocol that the hundred or so of raw bytes you save through omitting context, are most likely negligible through the prism of connection compression. So to me that's not entirely a "valid reason".
And as developer myself, I think that contexts, even in a non valid JSON-LD implementation, offer enough guidance for building a data vocabulary for them to have plenty of value.
Do you propose we replace contexts with Open API specifications, or how do we coordinate what's a valid vocabulary data object in a federated network? And how do you propose that others discover these specs?