The problem is finding a way to "objectively" tell the difference. Often, people say that a debate is "useless" when what they really mean is that it makes them uncomfortable. (Anyone who has ever argued about religion will know what I'm talking about here.)

I may be in a minority, but, at least on LW, I am strongly inclined to err on the side of allowing people to argue in public at length. I would be disappointed if an argument that I was following were removed from public view; while on the other hand those who aren't interested in a particular argument (or who are made uncomfortable by it) can simply choose not to read it.

One solution might be a "designated area" for hashing out disagreements. (I've already attempted to create one for a specific debate.) Maybe a monthly "argument thread"?