When I think about cultists in this game, I think about not tremendously tough, not tremendously damaging enemies that treacheries can turn into absolute landmines. The longer they’re around, the tougher they are to fight and the more doom they bring to the table. Diana's the opposite.
Diana, like Gloria, represents a class of mystics that no other class can really replicate. As mystics with guardian access, they can help the party survive with sheer denial. (It’s why both of them are probably bad in improv.) Miss Stanley, the investigator you probably care more about because you’re reading about her, doesn’t really fit into the clue seeker/flexible/fighter archetypes. She can do any of those things, but more than that she can simply ignore bad things that are going to happen. She can play defense in an amazing way.
In fact, putting Diane on the team makes more risky characters viable. Low willpower rogues like her cult buddy Preston, oddities like Calvin...They can spread their wings, knowing that the worst of the encounter deck will be taken care of for them, and that their weakness can be ignored at least once due to dark insight.