I wonder if this was really a bug or conscious decision? (Since they are saying it's a bad variable, probably the former. I'm too lazy to look into it.)
It's tough to say. They did seem to want to make this game accessible to everyone, so I'm sure they didn't want it to be too difficult. However, the unused behavior is how the Spinies behave in subsequent games, so I'm not so sure. Plus, they could have used this for the "second quest" where Goombas get replaced with Buzzy Beetles and all enemies move faster.
Probably no way to know at this point, and the increasingly senile Miyamoto & his motion control obsession likely can't remember this far back.