I've seen discussions of using the different weights of pins to subtly tune the top of the guitar. Some guitars suffer from a dead note around F# or even B caused by interference between the frequencies of the note and the top vibrating (a "wolf tone"). Adding or subtracting mass can help here; it certainly did with one of my guitars. My Yamaha had a weird overtone on the open B string when new, and the same note played on other strings. Changing to heavier bone pins cleared up the problem. I usually switch out plastic pins as part of the initial setup for my acoustics anyway. Bone is more durable.