"When dealing with people who you are depending upon to give you service it's best to treat them as they expect."
That's a situation where I would probably tip, Thomas. However I am still healthy enough to carry my own luggage to / from the airport and to / from my hotel room as well as other places.
If my taxi fare is $18 / $19 and I have a $20, I don't go searching for any extra nor do I wait for my change.
I'll be da--ed if I'm going to let people make me feel responsible for the livelihood of others.