Tires use an ozone and sunlight resistant additive, but you can bet that the valve stems do not. I'm sure that they are just plain old rubber, cheaply made, just enough to get by. I've seen them bent over at severe angles by inappropriately mounted wheel covers and trim rings, and knocked around by brushes when the wheels are cleaned. Metals stems are just a little more money but will last a lot longer. Other than price, are there any negatives?