Does a shield +X give the +X bonus without wielding it?

[Burley]

There's a general rule saying it doesn't work.
And a specific rule saying it does.

Unless you want general to override specific...

Nope, I don't. But, I also don't think the item description is a rule.
Sometimes an item may contain new rules, like the Animated Shield does. But, the three magic shields that use "hold," use it because it's more grammatically appropriate to use that word than wield (for weapons) and wear (for armor).
In other instances of Specific superseding General, the Specific is specific. In this instance, its not. It's simply a looser wording of the actual rule (which says a shield must be strapped to the arm to gain the magical effect).