composites can take more of a beating than metal can. Metal bends and can intrude on the internal components and can also impede the movement of the fixture. composites will maintain shape even if cracked. This is why bumper covers on cars are made out of urethane now rather than metal. It cost more for the urethane plastic but its cheaper in the long run for the consumer because it can take a beating more than metal.
A MX-4 falling from 15feet will prolly survive in one piece but something is prolly broken inside. i have 2 and they are tuff but the shock of hitting the ground from 15feet is prolly a little too mcuh.
Tell your client that he/she needs to use safety cables. A 20bls fixture falling from that high is not a good thing to have hit some one.