It looks like there's some kind of trend shaping up for posters to use 7-zip for multipart posts. Guess that could be due, at least in part, that 7-zip is free to use, unlike RAR. Now, I have a paid-for copy of RAR so I don't have a dog in that fight, but the problem that use of 7-zip instead of RAR causes is that Newsbin doesn't recognize it and won't automatically process the downloaded parts.
Any chance 7-zip unpacking might find its way into Newsbin sometime in the future?