One of the most striking aspects of the younger men in Fathers and Sons has to be their distinctiveness in light of what brings them together. Bazarov is the unifying figure between himself, Arkady, and Sitnikov, and with a philosophical figurehead you might expect a great deal of unity of thought and/or action. And yet instead we are treated to some fairly different persons. This isn't a loss at all for us as readers, of course, given that it would be hard to stand more than one Bazarov or Sitnikov. It also natural that a movement, even a nihilist one, is going to have both leaders and followers as a function of the unique skills and character of the various individuals. However, you have Arkady with his love of music and the outdoors, Sitnikov with his funny friends and generally awkward social life, and Bazarov with his scientific inquries. It can be hard to recognize them as pieces in the same movement at all.
At first I thought that this mix of characters was a result of Turgenev's moderate outlook between generations. He certainly seems to try and give the generation both its sympathetic and unlikeable folks. I've started to wonder though, if you actually need this mixture to get at the heart of what nihilism is and what it is not. If you have characters who believe (or try to believe, or claim to believe...want to believe) that there is no greater truth or value behind things, then it is always going to be difficult to choose actions for them. If you don't believe in anything then you are really free to act in any way whatsoever. If we only had one figure interested in nihilism, then Turgenev wouldn't have been able to show that that character's actions aren't the only option available to a nihilist. For that reason I think that Turgenev gives a more fair representation of nihilists by providing a couple different examples, even given that we lose two of them through the course of the novel. It reflects, to me, a deeper involvement in working out the philosophy of the younger generation than it might appear to at first.