It does little to detract from the purpose of the thread taken as a whole, and it's not feasible to expect moderators to intervene in every single situation on a per-post basis. Flaming isn't disallowed, but spam is. One flame post isn't spam, but it becomes spam when you engage the flamer and start a pointless back & forth exchange. That's what ruins a thread.
I also don't particularly want to foster the expectation that moderators are here to enforce each individual's interpretation of the rules, as long as that person makes enough noise about it. But even so, I'd be far more likely to consider your POV if you were to use the report functionality or send me a PM about it, rather than making half a dozen responses to the person and then come begging me to give warnings. If you ask me to intervene after you've already engaged somebody, I pretty much have to warn you both.
