What do you mean nothing in the real world precedent for what we see in Star Trek?
And to answer your question, armed guards are seen in pretty much every series while on duty. If nothing else, they at least have a sidearm. During the Dominion War, pretty much every officer had a sidearm while on duty, especially if they were close to or in the combat area (see DS9).
I'm not saying we "absolutely must have" guys wearing full MACO body armor, but it does make sense to have armed guards. It's canon, and it just makes sense.
To me, MACO's are the Marines of the Federation. I see them more as dedicated ground troops or at least better trained at it, but not necessarily special forces. Maybe that's how STO sees them, with all the armor they wear, but the ones in Enterprise certainly weren't Seals or Rangers. Again, I'm basing my opinion of MACO's on the ones seen in Enterprise, not the armor-wearing guys in STO.
You've taken me completely out of context. I'd appreciate it if you wouldn't deliberately try and draw such a long bow when is it quite clear that's not what I was insinuating.
You suggested, in your origional post, that fully armed and armoured MACOs should be strutting around Spacedock. There is absolutely no evidence in Star Trek of fully armed shock troops guarding installations.
At best, we saw red/gold shirts armed with sidearms, and very, very occasionally, carbines. There is no precedent as to why we should be seeing vac-suited MACOs around every corridor as you originally suggested. And honestly, if you mean to suggest that the real world's militaries act like Starfleet does with respect to doctrine and tactics, well geez, I'll just walk away now. Sure, there is commonality in the ranks themselves, but Starfleet is so far removed from current military thinking that trying to compare the two is anachronistic at best. Frankly, the word I'd choose would be "archaic".
Your opinion is that MACOs are Marines. Well... I disagree. Based purely on the roundel in the bottom right:
Why would we waste a valuable asset like MACO's, who are needed on the Borg and Klingon Fronts, when we have security flunkies for that? I don't view the MACO organization as a direct analog of the marines, but their expertise seems to lie in making things go boom, and general combat/high risk away team missions. These are not analogous to the internal security division which is specifically trained for protecting places like the Earth Space Dock. You wouldn't assign a military police officer to do what special forces do, would you? The only reason MACO would have to be stationed on the ESD would be if Sol System is on the front lines, otherwise standard starfleet security, which is more than likely a larger organization and better trained for day-to-day interaction with civilians (Of which I'm betting there's a large portion, Earth being the capital of the Federation and all) , is better suited for the task of defending the Federation against smaller, but no less meaningful threats. Odds are an installation like The Earth Space Deck has a more highly trained protective force, but I doubt in a society like the Federation's they're going to be actively showcasing really big guns and standing at every entrance. I'd argue all Starfleet security on the ESD should be removed save for a few roving patrols, guards outside Admiral Quinns Office, and a few at the transporter pad as well, and maybe a security center somewhere where they can monitor the goings on of the station. The Internal Sensors, and other internal security measures probably replace a large portion of the man force required, freeing up more Starfleet officers for higher priority assignments.
If we take these great blueprints as a guide, that is a lot of residential civilians (attached or otherwise to serving personnel). There would also be civilian traffic to consider. I think its fair to call it "mixed use" than simply military or civilian.
It might be an idea to attach some MACO to Admiral Quinn's office considering it's location of the promendade, but I'd rather not have them patrolling.
It is a time of war, but I imagine a set-up akin to how it was on Deep Space Nine where Starfleet had a hands off approach to internal station security, allowing Odo's police force and the civil magistrate system to deal with things. We also saw times when people of importance had a Starfleet security escort and/or posting of officers outside their quarters. re-Admiral Quinn /w MACO idea.
Enterprise D: A "Heavily armed away team" consists of guys with pistols only. Armed search parties equal four people, 3 pistols and one rifle.
MACO's on Enterprise: Grenades, rifles with scopes of variable sight mods and zoom, hand to hand combat tools...
That TNG security squad wouldn't know what hit them.
Long range, MACOs use their rifles+scopes, own the TNG guys.
Mid range, again, RIFLES. those TNG pistols are ****-poor accurate with ZERO sights. MACOs win again.
Short range/hand to hand. While TNG fundles trying to fight with their hands, MACOs pull out those batons and promptly beat the **** out of the TNG guys.
Oh, let's not forget MACOs, and Enterprise era security use rifles often, AND GRENADES. I'd like to see your TNG squad handle a stun grenade. Oh wait, they couldn't, because they don't even know what a grenade is it seems.
Hell, if we take the screen deaths as serious.... So far in my watching Enterprise, not a single MACO has died in combat. In TNG, frequently those security guys got killed off it seems.
Edit: I mean heck, pulling out a phaser rifle in TNG was "HOLY ****, THEY HAVE THOSE?" rare. In the opening MACO combat scene, they utterly own a group that'd give the TNG guys a good deal of trouble/long firefight.