Well, for one thing, it breaks the dueling fighting style. And I don’t mean it “breaks it” in the sense that it makes the fighting style overpowered, I mean it literally prevents the fighting style from functioning. “When you are wielding a melee weapon in one hand and no other weapons, you gain a +2 bonus to damage rolls with that weapon.” If your body is a weapon, you can never be wielding a melee weapon in one hand and no other weapons.
That's true only if you're not capable of any inferences of logic or reasoning
at all. That is to say, interpreting this rule is only a problem if you're a computer.
It's possible to
hold a melee weapon and not
wield it, correct? Say a sword that's sheathed and held by the blade, or you're grasping the wrong end of a mace, or even that you're holding a staff except it's as a spell focus and not a weapon. So there's already a difference between
possessing a weapon -- even if it's to hand -- and
wielding it.
It's very clear from the phrasing of duelist that you're intended to be only making attacks with one handed melee weapons to get the benefit. It doesn't matter what your other hand is actually doing, as long as you only get the damage bonus when you're only making attacks with the one weapon.
Furthermore, we should examine whether or not a given interpretation is broken or overpowered. Say we have a Fighter/Rogue with a shortsword and a hand crossbow. Is what the Fighter/Rogue doing -- holding a hand crossbow and never firing it -- a meaningful benefit? Not really. Would duelist fighting style be broken if it allowed a Fighter/Rogue to get +2 to his melee attacks while holding a hand crossbow, as long as he never uses it in the same round as he benefits from the fighting style? No. Not appreciably. He could just drop it, and he could say he has a leather thong so it hangs like a bandoleer, meaning it's no big deal to recover it. Does the game quality meaningfully improve by requiring these action gymnastics? Again, no, not really. We're not really adding anything to the game to interpret this fighting style as
intentionally preventing this interpretation for balance purposes.
Even if we find a situation where it
is overpowered, that still doesn't mean we need throw out every similar situation. Restrictions should be
narrow as possible, not broadly limiting.