I always tip bartenders, Gatorman lists good reasons. As a result, when I'm in a busy bar, I never have to sit around waiting to get the barman's attention..they'll look out for you.
If you work at a job that depends on tips you'll understand. I tended bar for a while, and it can get very frantic. I also drove a cab in Jersey City, NJ for a couple of years when I was young, and I made many observations about tippers there. The main one being...the more well to do the client was, the smaller the tip you usually get. I would take these executives to work, and they'd throw a quarter at you, or something.
Regular working stiffs know the value of a dollar, and are much more generous. I used to drive this one old black lady home from getting groceries. She would always give a good tip, and I knew she was poor. One time I tried to tell her not to give me that much, but she just said "no, if the other guys found out you were getting less they'd get mad at you". She lived in the projects, and it was kinda dangerous to drive in there, but she was old and had bags, so I would bring her right to her door. If one of those executives wanted to go there, I'd leave em on the fuckin curb.