I think this would be a great way to handle any "big" debate where two people agree in advance they'll duke out their positions, without resorting to the old "duck and run" once they've tasted a strong blow from the opposition.
Maybe you could even set up a thread for challenges.
In addition to the points you have outlined I think it would also be wise to allow participants to call for a judgement.
For example I have had times when I felt certain that my "opponent" was purposely ignoring or grossly misunderstanding statements I had made. After trying to correct this matter via more posts my opponent continued to state (s)he was addressing my points accurately.
While on the surface this situation should be awarded points (either me or my opponent was wrong), a "call" or "ruling" on the nature of the problem might be more appropriate. At the very least the judges could clarify that either one party was not making their point clearly (so there was a reason for misunderstanding), or that the other was not understanding a point which was clear.
In the latter case (if it persists) the "debate" could be called, because further argument (if one side refuses to acknowledge the points of debate) would be useless.
I also have a a suggestion on the point system (maybe better):
1. Substantive Point Advanced= +1
2. Substantive Point Successfully Rebutted= -1
3. Guideline violation= -1
4. Introduction of Point wholly unrelated to argument= -1
In this way the points accumulate for a debater as they are advanced, regardless of attempt at rebuttal. The original point scoring format seemed redundant on this point.
#4 encourages people to stick to the topic and penalizes them for bringing in arguments which do nothing to advance debate (often used too distract opponent). I don't believe this is actually a guideline violation, but does show poor skill.
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holmes