It sounds neat on paper, I'll give you that, but from a consumer standpoint it just doesn't make sense unless you're a hardcore gamer, and let's face it, most hardcore gamers back then were depending on their parents to buy their games. Putting out an add-on that requires the other system to run? If you didn't already own it, imagine trying to sell that pitch to your parents. It was almost impossible.
"Can I have this one system so I can play this other system that has different games from the first one?"