I will open with the fact that I am a bartender working in the United States.
I am paid $5.00 an hour in wages (this is $2.40 below minimum wage in my area) and work about thirty hours a week. My rent + living expenses such as gas and food cost me about $800 a month. In other words, my hours wages on a monthly basis are less than the cost to survive. I depend completely on tips to be able to make a living and afford any sort of luxury, such as this computer.
Don't get me wrong, on an average night I take home $50 in tips. But I depend on this fact, and customers that do not tip me do direct damage to what I can afford for that month. Now, I don't mind if I get jack in tips from opening bottles of beer. That isn't much effort, and if they leave me a quarter or fifty cents for it I look at it as a gesture of appreciation rather than reflection of skill of service. But, when I have to say make four Cosmopolitans an they walk away without leaving me so much as a dime, then that is simply rude bordering on intentional maliciousness.
A lot of people will tell me "oh sorry, times are rough with the recession and I can't afford to tip you" or something along those lines. I smile and assure them that it is alright, but this is the truth laid bare:
If you cannot afford to tip your bartender, you should not be spending money on alcohol.
This is the view from a bartender working in the U.S., hope it was insightful.