@irontoenail - For one thing, thickness for thickness, metal will usually provide a better resistance to force than plastic, so it is a better choice for that reason.
For another, I believe that it's extremely unlikely that the metal can be electrified by the wires. How would they fray inside the conduit?
And finally, a metal electrical conduit will help to stop electromagnetic interference with the wires, and stop them from emitting that kind of interference as well.
It is usually considered the best option for most installations, and usually even if the majority of the conduit is plastic, I believe that they coat the sides of them with metal for those reasons.
I'm not an expert on it though, so you might want to take it with a bit of salt.