That is, in any situation where the threat is high enough that you are willing to accept the risk of accidents, etc. you really want the willing attendees to open carry, either alone, or in addition to, a paid security force. (Even in a situation like airplanes, you still would desire volunteers to have appropriate weapons i.e. that wouldn’t pierce the airframe too badly.)
To understand why, we need to consider the typical threats:
One threat is one or more individuals coming in through the front door, openly spoiling for a fight, likely with firearms. The exact angles and distances matter a lot, however the basic idea is that if you are locked up with such an individual, you don’t want to have a firearm because the suspect can turn it on you, and you want your hands free to overpower the suspect while defending yourself. You would rather have your partner, not locked up with an assailant, wield the firearm. In that regard, a lone armed guard is in several ways suboptimal, because that guard is constantly interacting up close with the public; it may still make sense for that guard to carry, but we should bear in mind the tradeoffs.
The typical “mitigation” is to run a security screening station with multiple guards, so you can confront such suspects where your security force is. That mitigates people accidentally carrying (and therefore e.g. friendly fire), and helps with people who are trying to be clever in their murder plans, but it opens a different threat vector – people queued in the line waiting for screening, who now are completely exposed in mass, and likely shootable from distance. So against undirected killing, vs. targeted attack against specific workers, this approach has the reverse of the intended effect. It is not generally obvious that in a typical situation, a nonspecific personal checkpoint reduces the deaths. Moreover, against a targeted killer like a stalker, the checkpoint approach clearly increases the risk, because you know anyone coming out of that facility is unarmed.
An addition of open carry improves on the situation. If the single guard, or the checkpoint, is overwhelmed, there is defense in depth. If the murder spree starts on a different floor e.g. because of use of knives, smuggled guns, etc. then the open carriers clearly are in a better position to neutralize the threat. Nor does it create new vulnerabilities for the individuals coming in and out of the building.
The question would be: why do we think the open carriers are going to be effective in neutralizing threats? The direct answer to the question is basically irrelevant, because the open carriers and the paid guards are essentially similar in qualifications and capability for the task of taking on a hostile individual with deadly force. On the job, the paid guards fire their weapons even less than professional police officers. The paid guards probably make less than the open carriers, who can hold more lucrative jobs, and therefore can go to the shooting range more often. The open carriers, being volunteers, clearly have above average interest in the use of firearms, therefore are likely to wield them with skill. Where we might expect the open carriers to be less capable than a highly screened and paid guard (rare unless they are off-duty police officers) is in wrestling with a suspect, but as they are not stationed on the front line to meet a reckless attacker, that weakness is not as likely to be exploited as it would be for the stationed guard.