Not much advantage unless you intended drop and knock it. I have seen posting about rubber breaking and turning white, battery not working, LCD and button dropping out but hardly anyone talk about body frame.
In manufacturing terms, it is easier to form shapes with metal than with plastic. Metal can be bent, folded, stretched, reworked and annealed to give strength. Metal can withstand high temperatures. For camera bodies, nowadays magnesium alloy is used to get light weight and high strength. Coating is applied to give good outer appearance and prevent corrosion.
Plastics are not as workable as metals and are usually injection molded or compression molded. High strength plastics are brittle materials that can shatter on impact so if need super high strength, you need to embed fibres inside. That's fibreglass. But fibreglass is not cheap. Thin strips of plastics do not have high rigidity or strength in the thin direction so the plastic camera body is probably quite thick to get shape and strength.
Certain plastics degrade with high temperature. And ageing also take place and certain plastics will go from soft to hard, turn brittle and change colour.
That's what I can remember from materials engineering course. Your 500D might shatter and crack next time you drop it, change colour if used in the sun too long. :bsmilie: