Making the film more ensemble oriented is an easier way to draw in a larger audience, and using the main Justice League as a way to introduce the Dark team is a perfect way to maintain the cohesive and interconnected nature of DC’s current animated universe.
However, I think making Justice League Dark a team up with the better-known heroes is a smart move from a business perspective.
Some people may take issue with the fact that characters like Batman and Superman are taking prominent roles in a movie called Justice League Dark.
That frustration makes plenty of sense, as the Dark universe is already an alternative for people who have grown tired of the typical capes and cowls of the primary DC realm.