This argument covers many bases, but not the one of encounter with relative primitives vs. the use of forcefield armor.
After all, how could the primitives tell that the little box in Kirk's belt is in fact futuristic armor, capable of stopping their arrows, swords and cannonballs with ease?
In combination, the explanations offered earlier in this thread can
account for the total lack of anti-bullet, anti-shrapnel or even anti-arrow armor. Physical armor is very good (ST5 riot shields!) but cumbersome and does more diplomatic and operational damage than it is worth (speculation to account for the observed facts). Forcefield armor is feeble ("Slaver Weapon") and, while it can offer some protection from primitive weapons ("A Fistful of Datas"), is more appropriate as riot geat ("Homefront") and as a futuristic raincoat (anything from "Beyond the Farthest Star" to "Ambergris Element" or "Timescape") than as combat gear. Finally, hand armament is potent and can always be ramped up to destroy entire buildings or elements of landscape ("Frame of Mind", "Chain of Command" etc), so there's no point in trying to improve current physical or forcefield armor for combat purposes unless it can be improved thousandfold (speculation to account for the observed facts). For the range of low-level threats encountered in both modern combat and primitive ambushes (shrapnel, arrows), there exist some unobtrusive material solutions which we see in action ("Nor the Battle to the Strong"), but Starfleet prefers to resurrect the victims with modern medical technology rather than burden the troops with headgear or other obstructions.
We can always argue "It shouldn't be this way! Their tech should be better!" but that wouldn't account for the observed facts. It's IMHO only realistic to assume that several lines of technology and doctrine have been completely dropped when they lost in the rat race against their counterparts - and personal protection was among the losers, just like infantry armor and swords completely disappeared at one point in the real world, and only one of them made a comeback later on.