Observation at my nearby international airport (SJU): guardsmen rotating thru the outer terminals are MPs; no M16s, Beretta pistols at the hip (loaded and ready, I’m sure), individual radios.
Unit doing more of a cop-on-a-beat routine, seeing and being seen roaming the place, as opposed to just checkpoint lounging. Probably helps that PRNG is heavy with MP units, who know something about security.
Yes the main purpose IS to put on a show. Yet that IS one legitimate thing to do among the many that are possible. Just not even remotely enough if that’s all.
And BTW, the cliché of guardsmen as semicompetent “weekend warriors” more effective at seizing beer-and-barbecue joints only applies to some units. Some of the people in the SJU unit got back from cold, dangerous times in the Balkans not a year before 9/11.