The non-stacking thing is kind of annoying: kind of like being given a new ferrari and only driving it in 40 zones.
At least make it so that they stack for consecutive hits. ie. you might activate spirit sword x times in a row, which boosts your next x attacks.
At the moment it's way too easy to accidentally activate it twice in a row and miss out on the effects. Or, as way my case, make the logical conclusion that it stacks and be bitterly disappointed.