I wish the ground slam was more useful.
Why are the important keys Z and X? I kept pressing the Windows button.
Making the "spikes" not katanas would have shown that they are deadly.
The slam is perhaps not as useful as the other two upgrades, but it does play a pretty important role as a movement tool - especially against the eyes and the last boss, moving up and down is crucial. Or when falling right into a trap with no way out, but not being above it yet.
It seems pretty common for such games to use Z and X, but you're right, there could have at least been more options.
The intention for the spikes was not really to be katanas. They are sort of "facing" you - when they were completely vertical it was a bit odd to me that you'd get damaged when running into them from the side. I believed their spiky nature is enough to imply danger.