I'll be the first to admit that I don't know much about cycling, but I feel like this would promote an uneven leg workout?
(Edit) nevermind, I see how it works exactly now. So it WOULD help.
Because the cranks are not connected, a strong leg cannot assist a weaker one. This means that if you are riding both legs at the same time, trying to do the same amount of work, the weaker leg is being stressed more and it will respond to this stress and eventually become the equal of the stronger leg. This same principle holds for the fore/aft muscles. The pushing leg cannot help the pulling muscles on the back stroke. As one trains these muscles will become the equivalent of the pushing muscles.
(via website)
I suppose the reason I thought otherwise is because in one of the videos the one guy was just spinning one crank arm and having the other one sit still.