I can see this as useful, even so, arguments will still spark by an argument proposed discretely (flag) or openly (reply) against the receiver. Plus, by using the flag and proposing the context of your argument to the mod, you might even shift---or increase opposing audience---your argument from the initial debater to the bystander (moderator), IF they want to tend to (un)necessary quarrel. Also, your argument, once posted, is always going to find a way to publicize and offend.
Reasoned flags are useful, but morally flawed as it can leak strong/sensitive info.