What would adding crashes really add to the game?
Answer is simple. Nothing.
Yes accidents happen in real life. Do they include the entire coaster falling over and disembodying its riders? Generally no.
The fact is if you want to turn No Limits into a roller coaster crash simulator, then you would have to go out and make your own game. From what I can tell, the NL physics engine is based soley off of the track and the train type. It simulates no other physics other than how a train, which is really just an object that runs at exactly one point on the track, reacts to a LIM, climbs a lift, enters a loop, and moves through a block which is occupied.
Its a moot point, they arn't going to invest the time, energy, and money into making something that has no real use other than fueling childish behavior that can easily be substituted for the carage of a modern shooter video game.