It's not that retarded even in IRL uses (see: AA vehicles needing insane RoF to hit fast aircraft, pre cold war warships having plenty of ammo). Which points to a simple rule - if low rate of fire or heat limit of rate of fire is a bigger issue than ammo reserve, multiple guns make sense. And in most game mechanics, it does make sense, even if it's not realistic.
For the typical game that runs on health points, not simulating system damage, advanced accuracy, ammo, crew stunning and so on, the IRL effect of first shot advantage is lost, while being able to pump out lots of dakka is rewarded.
Meanwhile in modern tanks , the tank has something around 40 main gun rounds and can fire them off in around 4-6 minutes depending on the specific tank and loader without needing an additional gun. Obviously tanks like this hardly ever get much use of their maximum rate of fire IRL as that would mean spending waaaay more time reloading than fighting.
So, why squeeze another gun into the turret to get more peak RoF that's hardly ever used anyway? Better get 10 more rounds for the first gun.
Note that around early WW2, when tanks used relatively small ammo for 37mm and low velocity 75mm guns, sometimes they had one of each and loads of ammo for each, but once high velocity 75mm+ guns became common that went away and ammo capacity went down to 2 digits despite that due to the big powder capacity shells needed for high velocity, large caliber guns.
Yeah, you have the mixture of "you want to be as small as possible", since lower footprint, lower cost, excetera. However, on the other hand you have "return to scale" issues, where the larger you go, a lot of things make more sense.
IRL the general desire is for the lightest gun you can get away with, to allow the lightest tank you can get away with. Some people for example give the 88 on the Tiger tanks as a bit of bad tank design, because it was generally overkill, which forces a bigger tank, increasing weight, lowering life of the track.
So, modern tanks are generally the smallest vehicle that can practically carry a large enough gun: A T-90 vs a Abrams tank is only about a 40% weight difference, despite a T-90 being a much leaner, more cramped vehicle. If we had some bit of modern tech that made a 105 mm an adequate gun again, more compact gun and ammo would suggest using this to make a smaller and more compact tank, rather than adding more firepower. Since you can't really use much of it anways.
Boats as you suggest had different limits: a viable warship that can cross the Atlantic for example puts lower limits on scale: so, even if theoretically taking a battleship's 9 guns and spreading it across 9 ships would theoretically be more survivable than one ship, the lower limit of a naval platform that can support a 16 inch gun is probably higher than 5,000 tons, the weight of an Iowa (46,000t) divided by 9. And of course a Battleship has more weapons than the 16 inchers, counting the wiki in 1943 th Iowa had 157 barrels (mostly AA). 300 tons per gun. Not practical ship size.
I could see some plausible reasons it may be more advantageous to pile more weapons on one platform vs spreading across. Fusion vs ICE engines may suggest once your in a vehicle big enough to make Fusion practical in Battletech it makes more sense to make a bigger vehicle than a second fusion engine if that's the main price driver.
Different conditions can drive different trade offs too: A
Timber Wolf has 7 primary weapons. Each one separately would need a 40-60 ton vehicle to carry on modern terms. This would probably be more effective per weapon, but would also weigh 300-400 tons, vs 75 tons on the Timber Wolf. In the basically airborne roll a Timber Wolf operates, if your Dropship can carry 300 tons, you can drop 4 Timber wolves, or 4 vehicles with as much firepower as 1 Timberwolf.
4 Timber wolves now might be worth a whole lot more than 16 vehicles delivered over the course of the day, even if its less efficient per weapon. A jack of all trades that can muddle through most any situation it stumbles across may be better than lean perfected specialists that will destroy the enemy utterly, as long as the combined arms all stay in their perfect dance.
I've had similar musing on the Leman Russ. The sponsons are stupid in the anti armor roll: has big bits sticking out from the main hull armor for giant weak points that other tanks can engage from the front. Its profile is terrible compared to a modern tank. However, that very optimized frontal engagement modern tanks can design around are dependent upon good combined arms that can in some way limit flanking shots and rely on the presense of competent infantry.
If however your in a situation where flanking fire is going to be more common/harder to prevent, and effective infantry support is much less guarenteed, I wonder if trading off optimal tank fighting design for "organic" flank cover might not result in a net better survival.
Like, on the Russian advance, if the infantry just was not up to snuff, either in skill or quantity to protect the tanks, and coordination was as poor as it seemed: would a tank with 3 25 mm guns to give 270 degree view, and a large 120 mm+ large HE round, so accuracy is less critical. A more self contained vehicle less dependent on good communication or other people doing their jobs well.
May be completely wrong, but interesting thought non the less.