The more contributors to the "debate" the less chance of real discourse. We'll get just the repetition of sound bytes, rhetorical flourishes (from the more articulate) and cheap gibes. Bit like Question Time (or most of the "News" threads on AB). Arguments and premises will not be exposed to honest scrutiny and criticism. The usual response to a contrary view will be the (louder) restatement one's own position, or the misrepresention of one's opponent's position, or a sneer, depending on whether the moral or intellectual inadequacy of the "debater" is the stronger constituent of his or her character.
The US head to head format is also flawed with its artificial "opening statement followed by first rebuttal followed by...". I would prefer to see one to ones where opening statements are followed by a sit-down face to face real discussion which would allow the viewer to assess what differences between the two sides are genuine differences of principle or philosophy, and which differences of method only.