I understand them to be more than aesthetic appeal. I understand they are more mildew/mold repellent.
If someone told you that, they are either grossly misinformed or flat out lying.
Plastic jets have been/are the standard since the beginning. There has never been a mildew/mold issue that I've ever heard of. The Stainless wouldn't take care of that because the stainless is just sitting on top of the plastic. The entire jet is not made of stainless.