Buzzman,
They are indeed pieces of history, some in better condition than others. There are various groups that "adopt" a cabin and go to great lengths maintaining them at their own expense. And yes, you can spend the night in them, first come first served.
Buzzman, when the original Death Valley National Monument was set up, there were still people living on remote mining claims who were grandfathered in and continued living there. As they aged and moved away the claims reverted to the government but the cabins remained.
Today, as Sunman says, jeeper and off-road clubs still keep the cabins up. Some, like
Russel's Camp have running water, solar showers, and operating wood stoves, not to mention the views. When the Monument was expanded into a the Death Valley National Park, old BLM mining claims like the
Barker Ranch where included with in the park. Just outside Death Valley National Park on BLM land to the west is
Brigg's Camp where, if you bring your own propane, you can turn on the gas water heater or bake a cake in the oven.
Don't forget to raise the flag to let other wanderers know you have taken up temporary residence!