Well, I once had a 1932 Martin 0-18 that had plastic pins, and I got a set of ebony ones. Went from fabulous-sounding to fabulous-sounding. Could have been the new strings, too.

Personally, I think there is a slight difference, if only because, with a dense material totally surrounding the string, you may get better coupling than if part of the string is touching a less-dense material like plastic. Yes, the main vibration starts at the saddle, and the part of the string down in that hole isn't actually giving you the note, but if that part of the string is "fighting" the vibration, you can get some effect on the whole of the sound. Sustain, for sure, will be affected. Try getting lots of sustain from a tailpiece-equipped guitar sometime.

Electric players know the difference between a hard-tail (non-tremelo-equipped) and regular Strat, and both of them sound different from a Les Paul, which features a solid bridge/tailpiece as well as a glued-in neck (unlike the bolt-on Strat neck). Everything that has an effect on vibration has an effect on the sound. Some effects are more obvious than others, though.

Songster Bob Clayton