Courtesy makes you likable and smooths human relationships in everything you do. It gives you a huge edge. I don't understand why people don't do it. Maybe they grew up in an environment where courtesy is taken as weakness. Or maybe they are so convinced of their own merit that they feel there is no need to be nice to the "little people." Either way, they are seriously handicapping their future happiness.
Courtesy offers so much gain for so little effort. From smiling and holding the door open for someone at the supermarket to giving a considerate reply to someone who went to the trouble to come in for an interview, courtesy makes life better for everyone. The return smile and sincere thank you from the person at the supermarket brightens your whole day. The rejected candidate who received your thoughtful message might be the exact fit for a later job.
Similar to the quote in the original article, I believe you can learn a lot about a person by how they treat the hired help, whether it be at work or in a restaurant or hotel. If someone has no power of retribution, even if you are a selfish jerk, and you still treat them kindly, it's a pretty good sign you aren't a selfish jerk. On the other hand, if you are considering a future spouse or employer, and they treat servers and underlings like trash, someday they will treat you like trash too.
Courtesy is karma. You give to others, it always comes back to you.