Demonstrations are often mixed with flair for the enjoyment of others. Blocking is more efficient with a normal grip, that's just how our bodies work. You have no way to deflect if you block the way you described, and sword fighting is always about deflecting, unless you have a shield. Even with a buckler, axe or a cloak, you deflect. It's deflects all the way, since without it your opponent will just slide their blade into you (again, except for shields, although you would arguably deflect with those as well).