Polymer can be made into any shape imaginable, is lighter, easier and less expensive to manufacture, totally impervious to corrosion, and just as durable as steel when used properly in guns.
Not sure why everyone gets worked up about polymer guns. Wood - another non-steel material - has been used on guns for centuries. Aluminum has been in wide use in small arms since shortly after the Second World War. Polymer is just another material.
Guns are ultimately about hitting targets, assuming a shooter likes to use his/her guns, not just look at them.