I have been told that http://www.domain.com is always better for SEO than just http://domain.com. I was hoping that someone could explain why this is the case and explain the disadvantages of submitting a URL with only http://.
Because most people link to a site with "www". If I was recommending your site to somebody, or mentioning it on my site or forum, I would use www. If you have concentrated on getting http:// into the search engines, then my www links wouldn't be much use to you.