Well, if some mental illness is a prerequisite to being a Knight, then it's just another piece of worldbuilding. Besides, as has been pointed out, the illnesses aren't treated as 'diseases' here. They're treated as character flaws - Kaladin has a tendency to be fatalistic and pessimistic, to a point that he's clearly depressed. Mental illness can be a difficult thing to pinpoint, exactly. I have ADHD. However, there isn't some switch in my brain that says on-off. It's a result of every factor I've ever lived with to some extent. PTSD and memory-surpression are more clearly diagnosable, and have clearer causes, but just because it's possible to tell exactly how they came about and how they're an exaggeration of healthy, normal behaviour doesn't mean they aren't a real problem. It's like a slowly heating room; at some point, no matter that you could deal with every previous level of heat as it mounted, it gets too hot to handle.