What's the advantage of using mock_model() over stub() for a view spec? It seems like you would want to use the stub() if you are using a outside-in methodology since you probably haven't even generated your model yet.
With mock_model you'll get a mock-object that automatically has an unique 'id' defined, a 'to_params' method, and a 'new_record?' method (that returns false). Here's an example of the difference between the two:
@user = mock('User', :id => 10, :new_record? => false, :login => 'nicholas')
@user = mock_model(User, :login => 'nicholas')
Mock_model is a convenience, and as you point out in a view-spec where you might not have the model created yet it might make sense to go without. However, you'll see on the RSpec examples (rspec.info/rails/ writing/views.html) that they show them using mock_model. I think it's your preference. It isn't difficult to generate an empty model class as a placeholder for the future while you're writing your view. If I know I'm showing user information on a view, then I can probably assume I'll be utilizing a User model of some type.
Remember with a outside-in approach you want to go in and out. Dive in, come up for air, rinse and repeat. It isn't that you want to bang out your entire view in one session, then your controller then model. You want to work on smaller slices and iterate and increment.