It would really depend on the crew. Some guys find the SIM quite stressful, therefore they may not be that comfortable with an extra pair of eyes, however on saying that, some guys will be fine with it.
It will serve no real purpose for a cpl/ir holder to be there.
I have to disagree with this statement. I think it would be of great benefit for a CPL/IR holder to observe 2 professional pilots at work, dealing with problems as a team. It would give you a real flavour of what the multi crew environment is all about which is something you would not be used to, as all your flying apart from you MCC would have been single pilot.
It will also give you inspiration for the future and something to look forward to
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All the best.