Aside from its dismissal as a "stroke" practice by "da bubbas," the valve down arrangement provides a more protected and easier to reach position for the valves when underwater than the valve up one does, even with wings (the valves are well below the wing so the wing doesn't get in the way - and yes, you can still easily reach the isolation valve).
On the down side, hoses have to be custom made, and their routing can be a pain. And once out of the water the valve down arrangement is a disaster waiting for a place to happen. Just getting the gear rigged in the first place is a major pain - basically you have to rig it upside down and then turn everything over without dinging the valves and regulators to get the stuff on. And if you slip and fall???
On the whole, unless you have your own boat you can rig for supporting your rig while you don it and get into the water, or you're willing to routinely put your system in the water first and then get into it, I think the disadvantages of the valve down system far outweigh the advantages.
Rick
On the down side, hoses have to be custom made, and their routing can be a pain. And once out of the water the valve down arrangement is a disaster waiting for a place to happen. Just getting the gear rigged in the first place is a major pain - basically you have to rig it upside down and then turn everything over without dinging the valves and regulators to get the stuff on. And if you slip and fall???
On the whole, unless you have your own boat you can rig for supporting your rig while you don it and get into the water, or you're willing to routinely put your system in the water first and then get into it, I think the disadvantages of the valve down system far outweigh the advantages.
Rick