Doesn't seem like you can discount any sport, even if it's following is pretty regional, when it comes to the ACC Network. Syracuse/ACC Lacrosse probably matters, possibly even UCONN women's basketball could someday matter.
That said, it's hard to imagine that JHU lacrosse would have gotten the ACC Network on in Maryland. That's probably a bridge too far. Considering that most people in any given area are not even sports fans, then not college fans, then not ACC fans, etc, even if JHU lacrosse has a huge following as far as lacrosse circles go, I can't imagine that's enough. And even among the fanatics, nobody in the market is going clamor for a network when they don't have a dog in the fight outside that sport and season. Even if you are mostly a SU or UVA lacrosse fan, the network still has some value for you with football, basketball, baseball, etc.
Bottom line, you can't say that lacrosse is meaningless anymore, I don't think. But I'm pretty sure adding JHU lacrosse wouldn't have had any significant impact.