And maybe my impression isn't accurate, but he fit into the slot between the Nikolay/Pavel and Bazarov characters. He helped break down what I initially hated about Bazarov, because we see his process of learning that he can't get to Bazarov's state. And he's still young, so his "spongeness" seems natural, rather than existing because simply he doesn't have a mind of his own.

To be honest, I didn't get the sense at all that Katya was uninteresting. She was overshadowed by Anna, and Arkady got to "find" her, so to speak. Same way he learned not to take everything Bazarov said by rote. I do agree that Arkady was the thinking everyman, and I felt that that was what Turgenev thought a man should grow into.