The moral of the episode is powerful that humans need to evolve at our own pace so that our emotional, spiritual progress matches our technological progress.
What does that even mean? We have technology that creates food out of thin air and medical technology that can literally bring people back to life (which we have in today's world) AND erase selective bits of memory. When Riker wants to bring a girl back to life, everyone says no because... she needed to die?
pretty much sums it up. SFdebris has a terrific review of this episode, pointing out how stupid the moral is.
Why should Riker turn down the powers anyway? There's nothing inherently wrong with having powers or abilities, it's how you use them that counts. Picard and Riker both act like jerks in this episode.