It works as it always has. If you need to combine the masks, merge the channels before invoking the refinement. I never had any need for sampling all layers. Could it be useful? I doubt it because there is too much that could go wrong if e.g. different masks have different softness already and you'd erase those subtleties by using a uniform refine edge setting. More or less you simply need to adapt your workflow to be more suitable.
My current workflow involves creating of numerous layers to alter the original image and in the case if I need the mask of the final composition the only way to get it is to make a flat copy of all layers and apply Refine Edge to it. Not an elegant solution. So, Sample All Layers could be an usefull options, for me at least
I would agree with Mylenium. There is a lot that could go wrong and that would be very complicated to code. Best solution I can offer is to create a merged stamped visible layer shift-ctrl/cmd-alt/opt-E to create a single layer for you to refine your mask, then just move or copy it to wherever you need it.