I'm not saying the "spares" being in the cockpit is a necessity but if the capt can't deal with someone being in there and looking over his shoulder, how does he cope in the sim or on a line check?
Ah yes but...
performance anxiety is quite real among pilots! Sit there and watch the operating captain when you are augmenting (or checking for that matter). In most cases their behavior will be subtly different than when alone. I believe thepsychs would have a term - its kind of like "mirroring' behavior between individuals establishing communications.
You can deny it till you are blue in the face but it won't change human behaviour.