Agreed. Topcoat of leather and dashboard already have UV protection. Between that and the glass, any added UV protection is a gimmick at best.
Well, living in Florida I have seen many faded seats / fabric door trim even on cars only a few years old due to sun exposer. The Jetta has black cloth seats so it will show fading easily. The Llumar tint I am selecting does claim to block UV rays and (obviously) filters light so I believe it will in fact help to preserve the interior vs no tint.
Also, being ceramic it will reduce interior heat which has benefit. Our previous Jetta had the headliner fail at 5-6 years old, likely due to excessive interior heat during our summer months. Ceramic tint should help here as well.