Many people use the metal plate for looks now but originally it is used to reduce the chance of the rear bumper catching fire or melting, remember many heavy tuned turbocharged (or even N/A) cars spew alot of flames when driven hard. Plus its easier to remove the black burn merk off the metal then it is offyour paint. Sometimes they dont even use a metal plate but rather a fire resistant coating.
Personally I would rather my bumper not catch on fire or melt then worry about people that think it looks 'ugly as sin'.