That was my same concern, but I notice that both the sites mentioned have agents listed, so there are perhaps also listed on the REA site and the usual places with a link to the website. In that case, the website is just an extended ad allowing you to give a bigger marketing spiel.
Used as part of a marketing campaign, individual sites may have value, but used alone I think you would miss too much traffic from people who focus on their favourite RE website for leads.