It seems redundant to use an ARIA landmark if the most semantic and accessible way to define the main section of your content is the standard <main> element. Is adding role="main" to the element really necessary?
Not all modern browsers have already mapped the aria-role main to the <main> element (so far only Firefox 21+ and Chrome 26+ have mapped it). When all browser will support this feature, then the attribute role="main" could be dropped, but in the meanwhile it's better to still use it.
Sources:
http://html5doctor.com/the-main-element/#comment-35495 (and Steve Faulkner's answer) http://html5doctor.com/the-main-element/#comment-36407 (last citation and answer)