The 'danger' of a wooden return is if there is a fire and it penetrates into that area. If the blower is still running, the induced draft will fan the fire and blow smoke throughout the house as it is circulated. Course, the returns could draw smoke from the rooms around the house and do the same thing, but returns are often at the floor level, and smoke tends to rise, so that is minimized. I think they do make smoke detectors for the air handling system that can shut it down if it senses a fire that would stop that from happening, but not sure how many residential units have them, if any.