This common rule restricts users from engaging in "personal attacks" or "personal insults".
Seeing how some people turn intellectual discussions into a king of the jungle war - one might think this rule is a good solution.
I think it's not and here is why:
If a law came to pass forbidding the use of weapons - what do you suppose would happen? Criminals would still get weapons, illegally (since they, by definition, do not care about the law, but about getting away with disobeying it) and the police would not be able to use weapons to combat crime, since they DO hold themselves up to the law.
Crime would be unstoppable.
This applies equally to discussions. Words carry power with them - the power of ideas, knowledge - which result in behavior. Words, when the situation calls for it, ARE weapons - intellectual weapon against intellectual criminals.
When one is not allowed to use them against those criminals, the result is the same as in the case of physical weapons.
To illustrate my point, consider the following examples:
Someone posts the following: "Hi guys! I am really upset. X told me that I am cheating by doing Y! I am so angry. I only do Y occasionally, so how dare he accuse me of doing it?! To fight this injustice I have asked all my friends to stop associating themselves with him to teach him a lesson!"
The appropriate response to this person is by saying explicitly that they are dishonest and unjust. However, the "no personal insults" rule kicks in to achieve its goal of a "civilized discussion". No one is allowed to tell this person that they are dishonest. At best, people can say something like: "I don't think what you did is right"/ "I don't think this guy deserved what you did".
This gives the original poster appearance of rationality and honesty since such replies make it seem like the original poster made a mistake of judgment. The replies focus on whether or not such "judgment" was mistaken. The truth is, the original poster used no judgment at all - they used evasion and were thoroughly dishonest both in the actions they reported and in their reason for posting.
This "civil discussion" rule gives them the safety of posting any kind of dishonest act without being called on the immorality of their mental process.
How about this example: Two people are discussing a topic. One of them brings facts, arguments and logical analysis of the topic at hand and of the other side's viewpoint. The other side, however, replies without any attempt to digest what is being said to them - they barely even read what is said - all they are looking for is how to prove they are right, but without relation to the facts or arguments presented to them.
This type of debate is very common, yet the "no personal insults" rule forbids anyone from calling someone else on doing this.
Instead the rational side has to either quit the discussion quietly, or continue it AS IF they were talking to a thinking person (in the context of the discussion).
Having to absorb the intellectual corruption of someone else to try to translate it, in replies, into an appearance of an honest intellectual process is damaging to the honest person's psychology because it instills a habit of self-imposed deceit about the nature of corruption (e.g. "X is not dishonest, they are just misjudging", "X is not trying to secretly insult me, they are simply judging me wrongly", "X is not a non-thinking, self-made idiot, they just don't know enough about this subject" and so on).
Again, the corrupt intellectual behavior goes uncalled and unchallenged - and therefore, safe and secure, while the rational side is left to quietly carry the burden of the intellectual corruption of the other side. This tends to chase good people away from those forums or discourage them from engaging in an intellectual discussion.
The result is that it is very difficult to find people willing to think and discuss topics in online forums devoted to intellectual discussions. The good guys give up after a short while and the bad guys stay and enjoy the freedom to do whatever they want. If someone insults them in any way by calling on what they are doing, all they have to do is call a moderator to protect them.
It is important to make the distinction between those whose main goal in a discussion is to "appear right" and insult the other participants as a tactic of achieving this end, vs. those who pronounce honest criticism of the one they are having a discussion with.
The "no personal insults" rule fails to make this distinction; before this rule, policemen and criminals are nothing more than men with guns, which need to be equally penalized for it.
Ad-hominem is a symptom of irrationality, not an essential, just as the problem with promiscuity is lack of spiritual values - not the amount of sex, or the problem with hateful people is their irrationality, not the emotion of hate.
To come up with a good set of rules one must go through a process of induction to identify the essential characteristics of rational vs. irrational behavior. Only rules based on identifying the essentials of proper intellectual discussion will do to create a good environment for rational, prosperous, discussions.