From Andy:

I can think if a few cases where the algebraic rule would be non trivial, and you would need the rate of change of that value.

Say you are using the Guuy-Chapman model to define the charge in a double layer compartment, this would be:

q = n_0 exp(-ze(phi - phi_0)/kT)

and then your would also need the current which is dq/dt.

In my work, I use the Hemholtz model which is simpler, but I don't know if it will yield correct results or not.

In any case, here is an actual real use case of needing the rate of change of a non-trivial assignment rule.

I think Lucian is absolutely correct, it should be up to the simulators to decide for themselves if they (or their users) have a need for such features and decide for themselves if they want to implement this part of the spec or not.