Most people don't consider TDO as the key factor for gym defence, but the time taken to clear the defender. Blissey still takes more time to defeat than Metagross, Meganium, and any other non-legendary Pokemon.
I assume that whenever someone attacks the gym, they will defeat it and clear it. And people often have more than enough Potions/Revives, so they often don't care about taking damage. But they do care if it takes longer to clear, especially when they can just head to another nearby gym and clear that in less time.