I tip because I know a lot of people who had to live on waiting salaries, and it was not pleasant for them. They often had to choose between paying for heating or water etc, or getting food.
Yes, it's a problem that should be taken up with the government, but it isn't, and no one is rushing to either. Besides, I generally receive very good service, so why shouldn't I tip? I think a large part of my reception of good service is that I am nice in turn to them, because I like being nice to people. I am nice to them, which encourages them to be nice to me.
Having a quick read through the posts, I see a lot of people are hostile to tipping, but a lot of that hostility seems to be aimed at the people serving as well. People pick up on your unconscious cues more than you might realise. You walk in with the attitude that the waitor is just a method by which your food arrives, they recognise that, and subconsciously treat you worse. Act nice to people, and you may suddenly find that the service you receive mysteriously gets better, that's what I found anyway.
I don't actually eat out that often anymore, being now in the sort of financial woes that leaves me scrouging somewhat.