Solent - Although I understand your point you are missing a very important point. It is not about proving who is the super pilot that knows everything and can fly the single engine stuff to perfection, knows where to find things in the QRH in least time etc etc. It is about maintaining and hopefully improving flight safety. Its about t as in test and T as in Training. Yes, we have to tick the boxes and satisfy the requirements as set out by the authorities, but focus should be on training. Its about walking away from your LPC/OPC feeling that:
1. You learned something that you did not know before (technical, operational or perhaps about CRM)
2. You feel up to speed with things and feel confident about operating the aircraft and that you got answers to any questions that may have popped up since last time you did training/checking.
If you make mistakes, make them on your LPC/OPC, not in the aircraft. Its perfectly fine to screw something up in the sim. We take a moment to discuss what went wrong, making sure that the pilot in question makes a self assessment and comes up with what went wrong and how to fix the problem himself, rather than someone telling him what happened. Once everybody understood what went wrong and how we should deal with it, we do it again.
I know that there are authorities in Europe that do not allow LPC/OPC to be conducted in the aircraft if there are simulators available for the type of aircraft in question. This was the case in a previous company I used to work for where we used to do checks in the aircraft but the CAA stopped the company from doing it.
As Crosswind says, doing checks in the aircraft will always be more of a "tick in the box" type of exercise.