Why do some members believe churches would empty if a preacher periodically used Edward's types of messages reminding us we are under God's mercy?...

Perhaps
another take on this would be to bring out the notion of human fallibility.  One of the
challenges of Edwards' sermon is not whether he is right or wrong, or whether his tone is scary
of inviting, but rather how any individual can claim with such authenticity to speak in absolute
terms.  I am not advocating relativism or anything like that, but part of the reason why some
might turn away from such a message is that they have been betrayed by former people who claimed
to have such a strong and absolute level of "authority."  Edwards holds little in way
of ambiguity, and when addressing issues such as faith, the feelings of divinity, and human role
in a cosmic design, there seems to be more areas of doubt, reflection, and ambiguity than there
are of totalizing notions of the good.  This might be a reason why people might turn away from
Edwards' message, for individuals have seen the result of those who claim to have an answer, but
really do not.  Salem during its Witch...