Why does everyone assume Sazed can't actively use his Shards?
He is the vessel of Preservation and Ruin. If his corner of the cosmere faced a direct threat, what prevents him responding?
I would argue that Sazed could unleash the power of both Shards against the threat. It would be a Harmonious action in line with the intent of Preservation and Ruin. Protection of Scadrial, destruction of foe.
I think this is why Odium fears Harmony.