The big thing with Lightbox getting the A-League would be that it would involve the move into OSB, or producing of the content. A large part of the reason Sky has a monopoly over broadcasting NZ Sport is that they are the biggest player in content production, if Sky doesn't produce your sports content there is next to no content.
If Lightbox/Spark got into this area, whether doing it themselves or with a partner then they with scaling have the ability to provide competition to Sky in the areas that are where Sky does make their money, Rugby, Cricket etc as they can get the rights and do the production themselves.
This is what Sky's incentive to win a battle for the A-League rights is, if they don't get them they will find a threat in their main content when those rights come up again.