If you're not going to take the time to make your posts easily legible, then why should we take the time to try reading them?
Posts with poor grammar and formatting are literal eyesores. The eye strain caused by reading large amounts of text in one sitting, coupled with the back-lighting that most computers feature, can tire one's eyes out at a very quick rate.
That's one of the main reasons why breaking posts up into paragraphs is highly recommended, and required by English teachers. In addition to placing emphasis and focus on specific batches of information, it also allows the reader to take a quick break and give them a moment to absorb and digest the information they just took in.
The same concept applies to sentence structure. If your paragraph is one giant run-on sentence, it's just going to give people headaches as they try to figure out what you're saying. That's why you should include proper punctuation - it's the considerate thing to do.