One of the main arguments for Wanda having once been a predicamancer is that Banhammer would have had no need for a croakamancer.

I think you're forgetting what kind of person Banhammer was. He was concerned with philosophy. He probably had big discussions like what we saw in that one panel from Jillian's story. He didn't need to pop a bunch of soldiers because they didn't do much fighting (just the occassional merc job). So what is he going to spend his turns popping? If he wants to have philosophical discussions, he'll probably pop a wide variety of types to get a bunch of different viewpoints for his discussions. And this is pure speculation, but my guess is that casters are going to be, by their nature, smarter and more inclined to that kind of discussion than say a piker. So you want casters for intelligent conversations, but you don't want 20 foolamancers because then you don't have different viewpoints as such. So you pop a variety of casters. Hell, a croakamancer's way of looking at things could bring a very interesting new perspective to the discussions. And why not? It's not like he was in desperate need of anything else.