I suspect that new users follow something like an 90/10 rule. 90% of them are good to have around, and contribute more than they detract. New blood is good. But 10% contribute junk that is like virtual cholesterol, it builds up, clogs the system, and if left untreated eventually will be lethal to the community. of that 10%, perhaps 10% are simply toxic waste that you want to get rid of, and 90% just need encouragement to fit in better.
I sometimes comment on egregious comments by the 10%. You've reminded me that I should more often acknowledge the existence of the 90%, and on my hope that the 10% I'm looking at are part of the redeemable 9%.
There is an observation that goes back centuries, which applies here. The observation is that if you pack a barrel of apples and there is even a single bad one, the whole barrel will spoil. But if every apple is good, the barrel will remain good for the entire winter. Thus, "don't let a few bad apples spoil the barrel". We want the apples, but none of the bad ones.
Unfortunately the advent of refrigeration has caused us to forget the original wisdom and the saying is currently used as the exact reverse of its original meaning ("oh, it was just a few bad apples").
An interesting mechanism I think. Before looking that up I would have suspected a biological transmission of infection or something, not a chemical transmission.
In fact this is true for real -"offline"- life as well.