I think the idea of having a DH for CAT 3B came about for some XAA's thinking about the approach ban. If the RVR is at or above the required when passing the OM you can continue to DH. If, after OM, the reported RVR falls below minima you can continue to DH for a 'look see'. If there is no DH it means you can land with no decision to be made. I heard some felt uncomfortable with not having to make a decision. Having said that, a DH of 14' gives no time to decide. By the time you've thought about it the wheels have made contact and the speed brakes are out. But for a lawyer there was a point to make a decision. Fudge, fudge.