If you want realism you can look up the dark ages equipment and battle armoring. Heavy armor is not common especially with the mercenaries. Mainly used by horseman to carry all the weight, and rarely by any other attacking mobile force.
The reason why fatigue should be heavily taxed is very simply to restrict you from making a terminator with imprenetable armor who can freely jump around endlessly. Yes at the release I do hope it will be more stat based perks making a choise to be absolutely deliberate and as now just cuz why not. You should be doing choises, and there has to be a sacrafice for heavy armor no matter what. As already mentioned, even if you have no traits or backgrounds adding to fatigue, there are traits making it quite possible to use. For example in the attached file you can see a heavy armored guy. Full Helm -20 and Heraldic Mail -42(not to mention the weight in the bags -9). Yes he is a farmhand that gives him +10, but the main reason he is able to do it are the perks Brawny (-50% fatig from armor) and Weaponmaster (-20% fatig of attack). Again having a commander unit with Rally the Troops (on use replenishes fatigue to all in range members by 50% of users resolve) makes a big difference aswell. He still has 73 free fatigue due to high lvl, but it would be only 52 if not for perk. To reach the Brawny perk you need 3 points in tier1 so a lvl 5 brother can have this perk.
You can’t realy see on the ss but im in the middle of cycling out old members who were mostly low upkeep backgrounds with new ones, and only the low lvl new brothers are in light armor. It takes a while to be able to use even medium armors 100-200 not to mention the 200+. Before this most of my troops were in medium 180 armors with 200 helms. Now i’m trying the heavy approach with a key figure who should be boosting rally the troops every 3-4 turn to refresh the fatigue since most of my men have ~60. It’s just a test to see how it is without using defense skills like riposte spearwall shieldwall. Depending only on armor so to say while in attack only mode.
Ps. Sorry firs file has a mistake.