Submitting something to a forum is like sending an email; the people who read your message can't read your body language, and they can't hear your tone of voice. Most of us are not skilled enough as writers to always express sarcasm/kidding/whatever else in our posts; smileys help us do that and save a whole lot of confusion, and possibly offense. If you have a large post, or are trying to express a "compound emotion" (like:
) it stands to reason that you will have more than three smileys in a post.