The material used for bridge pins has become part of the folklore of guitar playing with some people preferring one material to another. In my experience its largely down to weight rather than whether they're plastic or brass. Some people have mentioned density but, for items of roughly the same size, weight and density are more or less interchangeable terms.

Experimentation has shown that if a bridge is reduced in weight for any reason, the upper frequencies of an instrument are reduced but if this weight is replaced; e.g. by using brass bridge pins, the upper frequencies can be enhanced. The moral of this is that if you want more top to your guitar use heavier bridge pins.

Those who are as long in the tooth as I am will remember the long lamented "Frets" magazine of the 1970s. They were fond of doing experiments on things such as this and, back then, people were adding brass nuts to guitars because it supposedly increased the sustain. It did indeed do so but simply because it added weight to the head of the guitar. They suggested that, if you wanted to try this for yourself, screw a "G" clamp onto the head as it would cost less than having a new nut fitted. As for tuners and capos which seem to be permanently fitted to many a headstock, has anybody noticed a difference?