I think that it is similar to everything in life - a double edge sword - it would be a great addition if you know what you are doing, but you might kill yourself (quite literary in this scenario) if you don't know how to wield it.
I personally find it quite useful, but then I normally watch the blackpool lectures series - you can hardly go wrong with that. I also watch my own video for improvement, others dances for inspirations and development of my taste, and sometimes just watch competitions and imagine that I am on the floor.
But if they don't know the difference and they feel they are improving who's to say they are wrong? They may not be learning how do dance in the current tradition, but they are learning their own way and perhaps it works for them - perhaps it even works better.
There's more than one way to skin a cat, as they say...
While I like your statement, I would also like to note that certainly there are people in the position for saying them being wrong - their dance partner, or whoever is dancing with them. I don't mean it as "why don't you lead? Why don't you follow?" type of arguments. But if that self learning person is hurting me, I am fairly certain that I could, and probably should tell him or her that you are wrong.