What is the danger of driving rusty cars if they become involved in a collision?

What is the danger of driving rusty cars if they become involved in a collision?
It depends greatly on the degree to which the car is rusted. If the rust is superficial and only on the framework, you risk greater exposure in a crash (less protection from penetrating objects through, say, a door). If the rust is extensive (the chassis itself) then the car is ultimately weakened, perhaps a great deal. In this case, it is quite possible that the chassis may catastrophically fail, resulting in an irreparable car, and new very compact you.