It's not a waste if you actually use it in the attack to make that one attack way stronger, and with the way stronger attack you manage to achieve something you could not with a weaker attack.
I've never seen him do it, or entertain the thought that he could do it if in need.
I've never seen any swordsman do it with another blade.
It's something i've only seeen with Enma, in the context where Enma is a special blade in terms of how it interacts with it's wielders Haki.
If this is proven wrong, i will change my mind.