Used the right way, stepping on someone’s foot can be effective. I did it and had it done to me a few times when I wrestled.
When people are “tied up” they’re not thinking about the possibility of their foot being stepped on; they’re thinking about throwing and being thrown, elbows, knees, head butting, etc. When someone’s close enough, you can step on their foot and push them backwards. The effect is like tying someone’s shoelaces together. When it happens to you, it’s embarrassing and you feel cheated. You feel like “how did he get away with something so simple and stupid?” Regardless of how you got taken down, you got taken down.
As far as actual stomping, I look at it more as a distraction than anything else. Think Sandra Bullock in teaching a rear bear hug defense in Miss Congeniality.