Devil's Advocate

Someone who takes the worse side just for the sake of argument

One who argues against a cause or position, not as a committed opponent but simply for the sake of argument or to determine the validity of the cause or position.

This term comes from the Roman Catholic Church, where advocatus diaboli (Latin for "devil's advocate") signifies an official who is appointed to present arguments against a proposed canonization or beatification. It was transferred to wider use in the mid-1700s.

http://en.wikipedia.org/wiki/Devil%27s_advocate