Aside from the lightning part people already said, he doesn't want to let too many people know his ability or they may know how to counter it before ever fighting him. If a block can suffice use it before revealing a trump card.
He blocks with he's sword because it has a special ability. Normally you wouldn't use a sword for defense but suigetsu's sword actually uses the iron in your blood to regenerate. So he knows if it breaks , it will just go back to the original state eventually. That's why!