The swords are powered by an Angel, what powers the Placard and other weapons? multiple angels, an Archangel, a god?.
I figured it was basically residual energy from the Crucifixion. I guess it could be angels too, though.
Harry can use them in accordance with their purpose (like the swords) but he can’t draw on the swords as a source of power for his purpose.
Well, Nicodemus was going to use the (other) Shroud as a power source for the plague curse, and even though that wasn't the
actual Shroud, it still had power due to being a focus of faith - yet Nic could use it for destructive magic that was very much
not in accordance with that faith.
Now, that might not work for the Swords, since an actual angel is probably consciously deciding what to do with the power.
But also, Harry isn't going to mess around with them like that; he's their custodian, but not really their owner. I really wouldn't rule out the possibility to use the Swords as symbolic items in thaumaturgy, even though Harry probably couldn't access the power of the angel itself. Just because Harry hasn't done it doesn't mean he couldn't.