As a SLF, I would not mind in the least a Captain exercising his judgement to allow (apparently in this case a nervous PAX) into the cockpit.
Great, but are there any other legal instructions that you also don't mind the Captain ignoring? This particular "nervous passenger" was returning from Finland. Apparently his nerves hadn't prevented him from flying out there? Do we all now use our individual discretion to allow nervous passengers on to the flightdeck?
If it helps the smooth conclusion of the flight, why not?
Because it is strictly disallowed. Willfull violation may well leave the company (who pay you to comply,) with their operators certificate withdrawn. It may leave them vulnerable to the withdrawal of their insurance coverage. It would almost certainly leave them, me and possibly other members of the crew open to the likely risk of prosecution. The repercussions may extend well beyond the simple act itself.
I have no problem with anybody wanting to start a revolution, fight communism, extinguish burning witches, tend to alcoholics, depressives, nervous flyers or any other cause. However do it in your own time, not when your employer is paying you to fulfill your contractual obligation to them.
is it not a part of that same trust which must allow him or her to use discretion?
Yes, discretion to act in a professional, mature, legally compliant way at all material times. That is the trust that the employer places in him, the passengers place in him, and just as importantly his crew place in him.