Some more information. The reason I think the MV target should have an effect is that I noticed differences in the simulation outputs when only changing the ManipulatedVariablesRate parameters.
For example, here are two simulation outputs (showing only the 2nd output variable, and the response to an input disturbance at t=5):
In one case (ManipulatedVariablesRate(2) = 5) this output is pushed up to its maximum constraint (red dotted line) and in the other (ManipulatedVariablesRate(2) = 2) it is not. I think this proves that there is 'slack' in the system - i.e. no unique steady-state solution to the MPC. Therefore adding the additional target to the objective function should reduce the degrees of freedom.