Brass bridge pins do only one thing really; they add mass to the bridge plate. Whether or not this is a good thing or not depends on the type of guitar and the method of construction. Some guitars respond very well indeed, with a noticeable increase in volume and a somewhat brighter sound. Others don't, and their sound may even suffer. I suppose it depends on what the guitar sounded like before, and what the player's idea of a 'good' sound is in the first place. I've tried brass pins on most of my guitars, but only use them on one currently. I've also done back-to-back comparisons and recorded them to try to take out the subjectivity, but the results were inconclusive at best; I guess my technique isn't consistent enough to produce truly viable comparisons. My current theory about this is that the brass pins add mass to the central area of the resonating body (i.e. the top) and thereby alter the fundamental harmonic of that body. This may alter which pitches are emphasised in the response to played notes, and thereby change the apparent tone of the guitar. Now, where did I put that oscilloscope...
Tim