Khafra, one doesn't need to be a moral realist to give impartial weight to interests / preference strengths. Ideal rational agent Jill need no more be a moral realist in taking into consideration the stronger but introspectively inaccessible preferences of her slaves than she need be a moral realist taking into account the stronger but introspectively inaccessible preference of her namesake and distant successor Pensioner Jill not to be destitute in old age when weighing whether to raid her savings account. Ideal rationalist Jill does not mistake an epistemological limitation on her part for an ontological truth. Of course, in practice flesh-and-blood Jill may sometimes be akratic. But this, I think, is a separate issue.