Should the cord on my miter saw have a grounding wire and plug?
In a two-prong device, if there is a short from the hot wire coming in from the wall to the metal housing of the device, the metal housing becomes hot too, which is a shock hazard.

The third (earth ground) prong simply returns current to earth in case of a such a short so the housing doesn't become hot. If this kind of short happens and you have a third prong, you'll pull lots of current momentarily from the wall through the housing down to earth ground at the moment the short occurs and trip the breaker. If you don't have a third prong, you become the conduit between the hot housing and earth ground when you touch the device, which could be long after the short mechanically has occurred, since the short is "waiting" for a return path -- i.e. someone touching the housing.

Anyway, if the saw has a metal housing, definitely replace the cord. If the saw has a plastic housing, the risk is lower. You can add an earth ground by replacing the power chord and tying the earth ground prong to the metal inside the unit. I usually do this with a ring lug if I can find something threaded inside the chassis. You could also clip the metal housing to something else nearby that is earth grounded (three-pronged), since earth grounds should, by definition, all sit at the same voltage.