I don't think they should rule him out too soon either, but I understand why it looks unlikely that GMH killed Cayle. There are witnesses coming out of the woodwork who have seen GMH on the trails and near other crime locations, but not one yet who saw him near Athens.
Also, I would find it hard to believe that he could do a "soft kill" with Cayle when he was mentally in a place where he had just finished murdering and dismembering Cheryl Dunlap and only two days before he did the same to Meredith.
http://www.ajc.com/news/content/news/stories/2008/01/13/athens0113.html?cxntlid=inform
"Investigators don't see any connection because there's no evidence that Hilton has been in Athens and there's no evidence of violence to her body," Clarke County coroner Bobby Tribble said. "There are no obvious signs of trauma."