Why does Clemency get interrupted? And why does Sword/Shield Oath reset combo?

[Synestra]

WHILE mitigating 20% damage AND hitting with a 300 potency attack.

Or simply hit Holmgang and take it to the face like a man(it also has same recast as sentinel so it is kinda "spammabe" when it comes to godmode CD's)and use those stacks on FC instead. Also is there even warriors out there that uses loldefiance for anything else except enabling the use of IB anymore? most just sits in deliverance and swaps defiance only for IB and after that back to deliverance pewpew.