In reality, though, the DT only system doesn't present much of a disadvantage for using high DAM weapons on low DT enemies.
Ding we have a winner! If the game balance philosophy is that automatics are only good vs low DT enemies at close range, one would assume that balance would dictate that high damage per bullet guns should excel at long range and be poor at close range against low DT enemies. The problem with DT is that there is a HUGE penalty for low damge per bullet, and no penalty at all for high damage per bullet against low DT enemies.
Assault weapons (many bullets, low damage) are only effective on low DT enemies, and only work at close range (high spread), and get less benefits from sneak attack criticals (low dmg/hit), and appart from the minigun, do not have a lot of dmg/ap in VATS. In fact, assault weapons in this game fill the same niche as shotguns.
The anti-materiell rifle does have some drawbacks on short range combat - long reload and high AP cost in VATS, and this is a good thing. That's a semblance of balance at least.
The Cowboy familiy, and the Brush gun specifically, are arguably the best guns in the game with cowboy perk and wadcutter ammo, work on ALL enemies, works on close range, medium range and can even be used on long range (low spread), has high dmg/AP cost in VATS, and produces astronomical sneak attack criticals. These guns are good at everything!
There's just no point in dragging around any automatics+ammo for low DT enemies when your good ol' cowboy gun will do that job AND serve all your other needs as well! Inventory space is quite precious in this game, esp. in harcore, and there is just no need for the automatics as long as they are limited to close range, low DT only.