ill weigh in on this as i just went through this.
hub rings carry no load its a fact, they are there purely to assist in locating the wheel and prevent it being off center.
doing your wheels up in a star pattern in several increments (20-lbs-50lbs -70lbs) will center the wheel due to the taper on after market wheels/nuts.
wheels nuts hold the wheel on and center once they are torqued, the hub ring will have zero load, only time it will be loaded is if the wheels nuts are loose/fail in which case since its alloy/plastic it wont do squat anyway.
so why have them? to minimize any chance of an off center wheel causing a vibration, that is it.
yes i run them i take the better to be safe than sorry logic but i know they are there to assist fitment.
as for alloy or plastic, i went alloy as thats what work wheels sell for the size i needed (60-54.1). i do track work so i agree that plastic could melt given the proximity to the disc and the airflow i think this is a small if none existent risk with high quality plastic ring, as meurz indicated he has had trouble free operation with plastic.
so if you track the car go alloy to be safe, if you dd the car plastic will be fine, but remember to torque your nuts to what toyota recommends 76 ft lbs, and re torque after 60-100miles (i do 100 kilometers) to ensure the nuts are tight when everything has seated, any tighter and your just going to weaken the stud, and it will actually not hold the wheel as tight as you will have already started the stud to yield.