I think that it's a good courtesy to read a thread before posting. Of course, when a thread is 40+ pages long, it becomes a bit prohibitive to read every single post. Television Without Pity forum has a rule that you must read the past 15 pages in a thread before posting. This is enforced. People have been (temporarily) gagged for failing to follow this rule.
Missing a point while skimming a thread is one thing. That's bound to happen and not really what I would consider an egregious error.
Starting off a post with the equivalent of, "I've not bothered to read anything but the original post (and none of the updates), but here is my opinion..." is really quite rude. Personally, I scroll right past those posts. It's the equivalent of saying "I'm not paying attention to anything you've said." If someone is not paying attention to relevant information, why should I read what they have to say?