Actually, you make a good point, there's no reason profanity must "stand in lieu of reasoned argument". In fact, a persuasive case can be made all the more impactful and compelling with a bit of coarse language, tactically applied to convey emotional context -- consider some of the content on, say, Cracked, or the Oatmeal.

So, on further reflection, I guess what irked me more was that the original post contributed nothing to the conversation, rather than the profanity therein; it's just easier to notice things present than things absent.

I suppose there's a "quality hierarchy" of sorts in my mind (loosely corresponding to upmod/abstain/downmod):

  - well-argued comments, in which case manners are incidental
  - poorly-argued comments, but at least they're polite
  - neither well-argued nor polite