Taking advantage of an Insane Investigator
In at least scenario 2 mythos and trauma cards are pretty cheap. For 1 threat you get 1 mythos or 1 trauma card per cultist. Playing the trauma card is in itself free. In my last game the keeper managed to get my insane investigator to run 3 spaces into one of her monsters who at once took a sample and ran through a locked door I couldn't follow. This ended up being probably the most important move in the game. I forced the investigators to go from passive to aggressive to stop the keeper from getting her last sample.  

So while the use will depend on the luck of the draw a well placed trauma card can change the course of the game. Many of the low insanity cards are really not nice. In a game where your objective is to kill the investigators you could get lucky and draw "final solution" and simply make the investigator kill themselves or the card that makes the investigator attack one of his friends.  

In a couple of the scenarios you get trauma at the same time as you get mythos. This is a way to get some use out of them.