[dragonwriter]

> In the particular cases being discussed, there's no ambiguity: "is a" means "member of" and "is the" means equals.

Yes, and fitting just those cases would result in a model that handled other cases incorrectly, because idioms inconsistent with that rule exist. (“Jodie is the bomb” has a meaning distinct from the individual words taken separately which is not stating a reflexive equivalency, for instance.)