I'm all for this. It's certainly just a polish, for one... And honestly, I think the biggest mistake they made with PROMETHEUS is that the best version of those two scripts was a combination of both. So a 3rd writer to polish the thing, and a good one, like John Logan, is hopefully, just what this needs.
I also don't think Ridley was totally at a loss for what to do with this sequel. Both writers of Prometheus mentioned having very frank discussions about where the series was headed,.. I just think that after that film was released Ridley saw the errors he made, and decided to correct them, so he knew he had to add stuff to the mix, and that's what all this has been about.
The script is the most important thing.