I think its rude to post without reading the OP.
BUT, (and I've just done it in this thread) I don't think there's any obligation to read every post in a thread before responding. Particularly if its very lengthy. often I simply don't have time.
I don't mean to pick on you personally, you're just the most recent person to say this in the thread. Here's what I don't understand about what you've written: If you don't have the time to read what we've written, why should we take the time to read what you've written. It's a lot like a conversation, it's not really a good conversation if you don't actually do any listening.
On reading the whole thread in general: I see it a lot like this: I have a class where the guest speaker speaks for the first twenty minutes, and then takes and answers questions for the next hour. I once had to arrive at that class forty minutes late. I chose not to ask any questions during the rest of that class because I didn't know what the speaker presented and what had already been asked. I felt it would have been rude to basically say "I don't care what you all have said on this topic, only my time and interests are valuable." I feel that not reading the whole thread works the same way. And sometimes you don't happen upon a thread until page 15, but if you're not willing to find out what's been going on in the conversation, I don't see how you can feel comfortable contributing.
I'm not saying I don't read anyone's replies. I do. But in a long thread, I may skip through it, looking for posts from the OP. Or read a few pages as I have time.
It also depends on the nature of the post. When someone's asking for opinions, my opinion as the 300th poster is just as valid as if I were the first.
If they're asking for advice, there's more likely to be additional info and comments throughout the thread.
And what about threads where it isn't really that important? Eg. the 'Worst movie you've ever seen' thread. Other than ideas, what everyone else has written doesn't affect my post.
Sometimes I have a post in mind which I think is really pertinent so I post it before I forget, and come back to read the whole thread later.
Threads aren't the same as conversations.
And re the bolded above, its a harsh interpretation, but are you essentially saying "If you can't devote the required time to the forum, your opinons aren't valid"?