Valid questions. In my mind, I would think the upside for the instructor would be getting free work done by me for the cost of instruction thereby increasing their productivity as say, two reserves could get packed at once, etc. And necessary busy work (cleaning,organizing, etc) that would be done, freeing them to work on other things.
To the second question, I wouldn't expect to work for you for free indefinitely after apprenticing under you. And that I wouldn't learn everything in one week. So the balance of learning new things and doing the other work would work itself out so that we both benefited.