Yeah I always thought this was weird. I haven't had much contact with the actual criminal system, but the way it's portrayed in popular culture makes it look all wrong.

Can it really be that you can trade punishment for efficiency (of the legal system)? Is that really why you're allowed to do that? It seems like a terrible time for people to be playing poker, yet that's what it looks like.

One interesting twist is helping the prosecution. If you turn in a fellow bad guy, by giving evidence that helps put him away, should you be allowed to trade in your own punishment for his? Presumably the same argument applies, right? Whatever you did was just as wrong, why should you get anything different? But you need some incentive...