In chapter 6, if you try to cut off the feral drugged pack after observing them, you’ll get attacked by Haken. A high vitality (or a very high combat) allows you to school him, which gives you a significant reputation bump from him.
There’s technically no absolute need to fight Rost. But if you’re not good at academics, intimidation, or diplomacy, talking to him’s not going to do much good.
And yes, as Saprook mentioned, curing Rost requires you bring up the possibility in the first encounter, and ending with Sonoma. The reason for this is that Sonoma’s faction are the only ones who have developed a cure for wolves going full-feral. The government would obviously rather isolate/jail the problem at this point, and General Rivera is more interested in fighting, so she actually appreciates feral werewolf killing machines.
I’d also like to note here that if Bly goes feral here in book 1, the same cure may have the potential to fix her…