I'd say the issue for people who are inclined to pick the first arrangement is one of trust. If you don't trust your employer not to abuse the second arrangeme

I'd say the issue for people who are inclined to pick the first arrangement is one of trust. If you don't trust your employer not to abuse the second arrangement then you'd be a fool to accept it. With the first arrangement the employer is incentivised not to exceed the agreed upon expected number of on-call weeks because they are financially penalised for additional on-call time. With the second arrangement the employer can demand as much on-call time as they think they can get away with. It's really a case of if you expect your relationship with your employer to be in some way adversarial - if you trust your employer not to screw you than go ahead, but you can't necessarily reliably make that judgement going into a new job.