The point as I understand it was to have the humans not have exactly our moral system. Morals evolve over time, and most people in any given generation would be shocked by the ethical and moral attitudes of people a few generations down the line. This attitude of the population reflects that. It also helps broaden the scope of the questions raised by not making one of the moral systems identical to our general moral system, so we don't immediately look at the morality of the humans and just say "but that's the right system!"

Overall, while I think I understand why Eliezer did this, it seems to be a very tiny benefit for a very large distraction. Overall, a net negative in getting his points across.