I think it should be in the rules, that posting to old threads is a no no. It's not actually considered a member rule... just poor etiquette. So, if I wanted to be an obnoxious ass (more so than I am already), I could just start responding to old threads all over the place. Granted, it would destroy any kind rep that exists, but I could point out "it's not considered a rule". Why not just add it in there under like "Keep it spam free"?
I just mean, it's kind of a pain to bark at people about posting to old threads (yes, I know about the cute little text by the post box... but they obviously don't.) when I have nothing to direct them to about their behavior.