No problem. The other thing it allows is for is other plugins besides Assignment to use the advanced grading forms. So it has to be agnostic about the user to allow for that.
For example, we have a plugin that uses advanced grading in certain circumstances and it uses a different table's id as the itemid.
I think the only thing your advanced grading form needs to do regarding calculating the grade is to define the get_grade method on the gradingform_<yourgradingform>_instance class.