Evil:
Many characters in JL the audience barely know exist. They are not as recognizable or know the basics like Batman or Superman. A JL film would define these characters for generations for the public and Hollywood itself.
The general public really is only unaware of Martian Manhunter, who's about as unlikely as any superhero to get a movie series, and Green Lantern. Four of the rest have had major television series/movies and Aquaman is well known, even if it's only due to Super Friends.
A JL film helping define these characters is only a bad thing if it's not a quality movie. And, IMO, we don't know enough specifics to answer that question. X-Men is leading to at least one solo movie, so there's evidence of that model working.
To me, JL is an easy sell. The World's Greatest Heroes team up. You can add "From the Award Winning Director of Mad Max" as a credibility enhancer too. And considering Brandon Routh and Tom Welling are, at best, B-listers, there's really only Batman to compete with from a credibility point of view. What you need to make JL work is to be able to sell it as an equal, but alternate, take on the characters that already have franchises.
FWIW, I doubt Christopher Nolan is going to walk away from Batman due to a JLA movie. He can probably name his terms and WB will meet them. But, no executive is going to let some director that doesn't have to answer to stockholders tell them what movies to greenlight or not greenlight. They may seek compromise and accomodation, but they won't capituate authority. Now, they may kill JL for other reasons, including concerns about audience confusion/acceptance in the market place/better alternatives, but they won't let a director dictate to them on their feature slate.
Edit: A complicating factor may be Christian Bale's contract. As far as I know, he, Oldman, and Caine are already committed to three Batman films and Christopher Nolan may not want to abandon them.