If a thread has 6 regular posters and 5 of them never discussed anything in the previews (because they skipped them) and only one watched the whole episode, I reckon the thread wouldn't have much in the way of a preview discussion. But if half of the 6 watch the whole ep and mention the preview occasionally the same as they might mention any interesting detail in the ep, then they do it because it's fun and they enjoy watching the previews. This forum is all about entertainment and the anime (and manga, books, movies, games) threads are meant to grant additional entertainment after watching the episode (except the SAO thread, which was meant to relieve some stress accumulated during the ep). It would be kind of a pity to remove that extra entertainment.

Rules, traditions, and conventions are always compromises to a degree. Sometimes you are lucky and they suit you, sometimes you aren't.