Getting paid is an important necessity in life. There is a popular blog called "clients from hell." Whenever I read that I'm always so surprised that more designers/freelancers/whatevers don't just learn from those experiences and make better agreements up front with their clientele. Most client issues can be solved ahead of time with the proper preparation. The sad thing is that most of us end up learning the hard way.
Now - it is also important that there are some clients you really just shouldn't be this harsh with. Non-profits, for example, should always be offered pro-bono service as it is our obligation as a creative economy to share our knowledge freely with those that do not aim to profit from our work.