Why? I upgrade (be it guns, cars, scuba gear, ect.) to improve reliability, function and sometimes just because I think it's better. If the steel rod wasn't better than a plastic one why would Kel-Tec use them on the P-3AT which is the exact same design? The plastic ones probrably cost them less than a penny each, the steel ones maybe 5 cents each. Build 100,000 pistols with a plastic rod they make $5,000 more net profit. Same logic the car companies use.
If everyone had the "if it ain't broke why fix it" attitude we'd all still be shooting Colt Peacemakers and driving Model T's