Just had a reply from ASRock Support:
Hello,
I don't care what asrock support says, the molex power connectors aren't to be used UNLESS you are using Unpowered risers, not powered risers.
I have both the older version of the h81 and the newer r2.0 neither of which do I power the molex on the motherboards and they work as intended
I have only one of these boards - and i't been up and running for 4 months. I am running Windows 7 and 6 1070ti on it. I'm using powered, Mintcell risers and found it doesn't matter if you plug the Molex cable into the MB or not. Works fine either way. I'm not sure why you think you shouldn't use it. I actually have it plugged in right now. I don't think it matters - no harm with or without. But I am interested to hear why it should NOT be plugged in or are you saying it doesn't matter if it is or isn't?