On one of our cruises a couple of years ago, the CD referred to himself as our "Director of Entertainment." I was floored, initially. Then, I realized it was true - not only on our ship, but in general.
But, he was much better than one who we had on Celebrity...the lounge lizard type, who also encouraged us to "make some noise...make some more noise.." Not quite what I expected from Celebrity. And, as a tie-in to Dave's note, he actually was a TV host. He showed us numerous clips.
My all-time favorite was Tom Dentino on then-Dolphin Cruises. He interacted with the pax, got to know our names, and was always visible. And, he knew the ship, and could answer any question. I wish there were more like him.
I do have to say I've never sailed on a ship on which Ken Rush was CD, although I did see him on several Travel Channel specials, and he does seem to know his stuff as well as have a genuine enjoyment of his job.