You also have to calculate what the effectiveness of your suppression is. If that effectiveness is negative, as is plausibly the case with hamhanded tactics, the rest of the calculation is moot.
Also, I believe I have information about the supposed threat. I think that there are several flaws in the supposed mechanisms, but that even if all the effects work as advertised, there is a factor which you're not considering which makes 0 the only stable value for the effect-if-dangerous in current conditions.
I agree with you about the effect-if-not-dangerous. This is a good argument, and should be your main one, because you can largely make it without touching the third rail. That would allow an explicit, rather than a secret, policy, which would reduce the costs of supression considerably.
Some research says that lurkers make up over 90% of online groups. I suspect that Less Wrong has an even higher percentage of lurkers than other online communities.
Please post a comment in this thread saying "Hi." You can say more if you want, but just posting "Hi" is good for a guaranteed free point of karma.
Also see the introduction thread.