So for cruises that do not require a passport the common theme you'll hear people say is that while you don't need one, they are great in emergencies.
I am a fan of having the passport only because it makes life easier at check in, etc...
However, if you really are part of the group that wants to have your passport rare event that you end up in a hospital while on an excursion or something happens and you need to fly home unexpectedly then it does you no good to have your passport for this reason if you then leave it on the ship.