I would tell the customer this:
Plastic is inherently SAFER. There is no way for wires to energize the box, then the yoke, then the little screws on your switchplate which, if the ground wire malfunctioned, would carry 120V and shock you. Because metal boxes are "likely to become energized" electricians have to spend extra time to ground them to protect the final user.
Plastic boxes have more space inside for air flow and cooling of wires which results in a more efficient system.
Because plastic boxes are larger inside and "breathe" better, the expensive devices like GFI outlets and dimmer switches install easier and last longer.
Metal boxes still have their place, but in my expert opinion plastic is the better way to go most of the time.
Just say it with conviction