NTLS and Sprint's relationship is already strained. Sprint has already said they are looking into expanding into NTLS service areas. Their deal end next year. Even if Ergen bought them, their is no guarantee Sprint re-ups that deal. They could easily just tell them both to kick rocks and do their own build out.
Also NTLS is a regional carrier with only regional spectrum in a very rural area of the country. It might be a stepping stone, but it still will not help them get that national network, which they need for their build out requirement.