So - they act as information providers
Also, they are a deterrent to car break-ins... There is HUGE trouble with break-ins at some crags.
...but maybe getting a different perspective on the topic might make that "yeah!" be more of a "hmmm...." and think about it themselves.
I'm all for open minded, open ended discussion, but when I read your posts I feel like I'm being force to drink Mohonk flavored Kool aid.

Information providers -- you could say a police officer patroling the streets is the same, but that's not their
primary purpose. The fact their positions are at key entrance points is clearly an indication their employment isn't about helping others, it's about checking for passes. Helping is a distant secondary outcome. It's very obvious to me at least.
Break-in deterent -- really? How would they know who's gear/rack/stuff is who's? Unless they see the break-in first hand, this isn't a valid reason. And for them to see anything they have to leave their post, which they don't do until sometime in the afternoon. The only parking area the guard/sentry can see in any detail is the smaller lower lot (near the stair master), and even half of that is out of view. Plus they have the usual "we're not responsible if someone steals your stuff" clause, if they had any real interest in guarding the vehicles they wouldn't have this disclaimer.
BTW, where are these problem break-in crags located? I've heard of one incident in 4+ years climbing, and by all accouts that was noted because of how rare break-ins are. Not saying it can't, won't , or doesn't happen, but if you're making a case for the guards serving a dual purpose there needs to be some substance to your claim that they're even needed for that purpose.
These are great examples of why I doubt anything you say to be the truth (it might be what you believe, but it's just an opinion and comes across as the official Mohonk "shpeel" to keep the flock in line). I'd wish to hell
someone from Mohonk would grow a pair and admit they have a large interest in securing their income (and while at it admit climbers are a CRITICAL portion of that income -- in other words, show respect and appreciation to those keeping the lights on). Instead you dance around this, appealing to people's emotions and sympathy as a smoke/mirrors tactic to avoid getting at the heart of the matter.