I think it's disgusting. Each case needs to be looked at individually. And I'm sure in most instances, there may well be another patient waiting that can take up the place of the other patient. That way, if you are running late, you can just swap time slots.

People that genuinely being rude and not cancelling - they need to be noted though. An appointment that they choose to not make, costs the doctor in turnover and it is a time-slot that could have been allocated to another patient on a cancellation list.