If you have a Mobile Share Value Plan, but don't have any phones on Next yet, you still got the $15-$25 discount on each line, depending on the amount of data. If you currently have it, once you get a new phone on a two-year contract, that discount will go away. The only way to keep the discount is by buying a phone off-contract or outright, or buying with Next.
Exactly - here is the math as I see it.