A few random comments on this topic:
We tip, but the people taking our bags usually seem surprised/gratified to get the tip. Based on that reaction and the comments in this thread, there are lots of good reasons to tip and to not tip. I have never felt pressured to tip, except sometimes in one of the S. Florida ports (that's why I qualified the first statement with "usually").
We just sailed Radiance out of New Orleans in December. When we were heading back to the airport after the cruise, we were surprised to see trolleys with baggage for the next cruise on a side street and unattended.
When we sailed out of Amsterdam on Royal, we put our baggage on a conveyor belt as soon as we arrived and that went right into the terminal.
We used to sail Disney and had a lengthy discussion with the gentleman taking our luggage to the car when we sailed out of Port Canaveral. That was when I first found out about longshoremen, as he was one, and he described their various assignments related to handling the cruise ships.