it depends on other practicalities, such as skill level of the pilot (particularly in the early learning stages).
Maybe - Maybe not. Assuming the instructor has the ability to first demonstrate to the student the correct width of the downwind leg along with required turning points, then the student has something to aim for. But if the instructor lets the student fly the circuit he feels comfortable with, then that is poor instruction because that could impact other users of the circuit. A learner driver would probably be happier driving down the middle of the road rather than keeping to the left. But most driving instructors wouldn't allow that to happen.