Had some more thoughts on this: The hollow parts in bones with gradient infill might also be used for cables, air transport, or signaling with lights.
I've got bad experiences with broken cables on headphones, so I'd like to avoid bending cables. I thought the bones cold have metal parts at their joints, to be used for data transfer or electricity distribution, the cables or metal rods would be inside the bone. Alternatively, data transfer with light signaling, which might be less reliable, though.
If a bot uses air pressure for air muscles, then bones might transport the air in some channel, so only smaller parts outside need to be sealed of while being flexible. This might even be usefull to stretch a extremity. Joints would be bendable without pressure inside, but not if there is pressure. Also unbend when the joints are filled with pressure.
Even if it's not done that way, there's something else: Air muscles might make ugly noise bc pressure release. This might better happen within the bones, so it would be less noisy. Though with enough other material around this might not be so important.