I should add that I would not have known how to practice solo had I not spent lessons asking my teacher to show me how. I believe you that there could be counterproductive ways of practicing solo. And I am sure I could do better at solo practice. I could learn more ways to check if I am executing actions properly.
Dancing with my partner is not always going to tell me if I am doing my part correctly either. Dancing with my teacher, sure, a miss-step is my doing. But with my amateur partner it could be either or both of us. And I am sure some of our errors go undetected by both of us, just as they might solo.
I do agree it is important to practice carefully so as not to practice wrong.