Stephen Batchelor in Chapter 14 of Confession of a Buddhist Atheist nicely summarizes the problem with mystical beliefs:

What is it that makes a person insist passionately on the existence of metaphysical realities that can be neither demonstrated nor refuted? I suppose some of it has to with fear of death, the terror that you and your loved ones will disappear and become nothing. But I suspect that for such people, the world as presented to their senses and reason appears intrinsically inadequate, incapable of fulfilling their deepest longings for meaning, truth, justice, or goodness. Whether one believes in God or karma and rebirth, in both cases one can place one's trust in a higher power or law that appears capable of explaining this fraught and brief life on earth. One assumes the existence of hidden forces that lie deep beneath the surface of the contingent and untrustworthy world of day-to-day experience. Many Buddhists would argue that to jettison belief in the law of karma—a scheme of moral bookkeeping mysteriously inhering within the structure of reality itself—would be tantamount to removing the foundations of ethics. Good acts would not be rewarded and evil deeds not punished. Theists have said exactly the same about the consequences of abandoning belief in God and divine judgment.

^z - 2010-05-02