I've always smoked my ribs meat side up and haven't tried anything different but I probably need to for sake of argument. It seems to me that meat being in direct contact with the grill would lead to the meat drying out quicker. I understand the concept of the concave part holding juices and moisture but it seems like the moisture would be better suited on the top side of the meat and not the bones. Besides if the meat is facing down, it seems like it would knock all your rub off but it would also makes it kinda hard to spritz with juices, add bastes, finishing glazes or jellys to the meat.
I might try Pitbull's advice and start them off meat side down and then flip them to finish.
Don't get me wrong I'm not saying BW or Will are wrong in the way they smoke their ribs I just haven't tried them thata way.