Context loss via Implicit Information
A post model is not a view model. A form view model is a subset of the post model. Even when they are structurally the same, the roles are different, and it's highly likely that changes will exert a pressure to evolve the details of those roles separately.
Here's the problem: If, at the point that your view model and your post model happen to be structurally identical, you decide that it's "overkill" or "duplication" or "unnecessary complexity", and just use a single model, then you make those two roles completely implicit, and model a scenario that is only circumstantially true. A less experienced developer will come along, and absent the experience to know that there are two roles in one place, evolve the single object in-place rather than split the roles out. They don't know any better, and you have hidden from them the information that would indicate to them the better way forward.
In short a data object is incomplete without its context. Its responsibility, in terms of the Single Responsibility Principle, is naturally coupled from its context. This isn't a bad thing, as long as you recognize and respect that a data object has no standalone identity.
So how do you respect this symbiotic state of affairs? In a strongly-typed language it's very simple: by giving it its own, layer-specific type/class. This way it will become blatantly obvious when a data object has gotten too far from home and is in danger of being misused or misinterpreted.
In a loosely-typed language, you respect the coupling by cloning or mapping a data object as it moves between layers. Since there's no type system to stop you, it's extremely easy to just pass the same thing around until a clear requirement of change appears. But the very fact of the data object's implicit role means that the developer will have precious few clues to this. By always cloning or mapping into a new object the potential damage is mitigated, and the developer gets a clue that something is being protected from external modification. Hopefully this distinction of identity encourages the developer to make even small changes to structure and naming that ensure that the object is always expressive and clear of purpose relating to the context of its usage.
Recognizing this is a nuanced design skill, and it's one that people often resist in order to avoid adding more types to their programs. But it doesn't take long to reach a level of complexity where the benefits of this care and attention to expressiveness and signalling become clear.
Here's the problem: If, at the point that your view model and your post model happen to be structurally identical, you decide that it's "overkill" or "duplication" or "unnecessary complexity", and just use a single model, then you make those two roles completely implicit, and model a scenario that is only circumstantially true. A less experienced developer will come along, and absent the experience to know that there are two roles in one place, evolve the single object in-place rather than split the roles out. They don't know any better, and you have hidden from them the information that would indicate to them the better way forward.
In short a data object is incomplete without its context. Its responsibility, in terms of the Single Responsibility Principle, is naturally coupled from its context. This isn't a bad thing, as long as you recognize and respect that a data object has no standalone identity.
So how do you respect this symbiotic state of affairs? In a strongly-typed language it's very simple: by giving it its own, layer-specific type/class. This way it will become blatantly obvious when a data object has gotten too far from home and is in danger of being misused or misinterpreted.
In a loosely-typed language, you respect the coupling by cloning or mapping a data object as it moves between layers. Since there's no type system to stop you, it's extremely easy to just pass the same thing around until a clear requirement of change appears. But the very fact of the data object's implicit role means that the developer will have precious few clues to this. By always cloning or mapping into a new object the potential damage is mitigated, and the developer gets a clue that something is being protected from external modification. Hopefully this distinction of identity encourages the developer to make even small changes to structure and naming that ensure that the object is always expressive and clear of purpose relating to the context of its usage.
Recognizing this is a nuanced design skill, and it's one that people often resist in order to avoid adding more types to their programs. But it doesn't take long to reach a level of complexity where the benefits of this care and attention to expressiveness and signalling become clear.