No. The dual-wield trait is not useless.
If you have a weapon in your main hand that is not heavy, you can use a hand-crossbow in your off-hand to make a bonus action attack. Dual-Wielding as a weapon trait does not specify that you need to be wielding melee weapons:
Now if you check the Two Weapon Fighting section, it does specify that you must use melee weapons.
This is where specific beats general. The Hand Crossbow breaks the Two-Weapon rule by allowing you to use an off-hand crossbow... but.
Crossbow Expertise breaks both rules.
In addition to the loading quality removal, and the lack of disadvantage, you are no longer bound by the "Heavy" rule of dual-wielding so long as you are using a hand-crossbow in your offhand. You are simply allowed to make an off-hand attack if you take the attack action while holding a hand crossbow in the off hand.
While there are no one-handed heavy weapons within the Adventurer's Guide, you can get into various situations where a Narrator could allow you to fire a heavy weapon one-handed. Such as a Heavy Crossbow mounted on a tripod, or a broken wall. Or perhaps a giant wielding a human-sized greatsword in one hand and a hand crossbow in the other.
And, of course, a Narrator or Designer could create a one-handed heavy weapon,