i only farkle for utilitarian purposes--mostly armor. skid plates, radiator guards, lights that are less likely to be broken, etc.
if i happen to find a good deal on a used whatever, i will go that route, but i don't spend much time on it. that stuff all gets put on a bike as soon as i buy it, and i won't really ride without it (because i don't want to have to buy new radiators, etc.), so i generally want the stuff right now.
i can understand why people like to farkle just for the sake of farkling, though. makes it more "your own", it's just fun waiting for shiny new stuff to arrive and opening the package, etc. i was actually kinda happy when i broke the orange front fender on my KTM. gave me an excuse to buy a black one, which i just like better for the looks and because it is "different" from the rest of the sea of orange even though it provides no additional utility.
i can also understand why some people might find a farkle to be useful that i would not (different seats come to mind...seems to be the first thing a lot of people switch, but i personally have never had a problem with a stock seat).