I'm going to list a few reasons for why we may not need this movie
1. What if it is bad?
The JLA is arguably the greatest superhero team of all-time, from its cast of characters to its great stories. This movie will have to do justice to the characters. What if it doesn't? How would we react if one of our favourite properties was absolutely ruined? (ala Green Lantern). You heard the outcry from Green Lantern, the Transformers movies and the upcoming Alien Turtles. The JLA arguably has a higher fanbase than these properties I just listed as it has to do justice to at least 5 characters. How would we react if at least 5 properties were ruined at once, let alone 1?
2. The Animated Series
No-one can deny that JLA TAS is one of the greatest animated series based on source material. Somehow, JLA predates the Avengers movie. JLA created a Batman and Superman animated series, established Green Lantern, Flash and Aquaman's universes and crossed over often. Then, they created Justice League TAS which surpassed our expectations. These animated shows created a whole other DC continuity, why do we need this in live action? Are we possesed by the novelty of seeing our favourite characters played by real people. We have our definitive DC universe in another medium, why do we need another?
3. Is a singular movie a good idea?
From what we've heard, DC wants to start the DC Cinematic Universe with a Justice League movie with nothing leading up to it (except maybe MoS but we have facts to debunk that) is this a good idea. Normally, the JLA comic is supposed to be the characters getting together and punching stuff and the character development is meant to be in the characters solo books. We've seen releasing JLA first resulting in a major disappointment. Geoff John's Justice League was a major disappointment to a lot of fans because it was released first, its first story arc was finished before all of the New 52 was out. People complained about its lack of character development.
Conclusion:
I think that we'll have to see if it is a good idea, I still remain optimistic, but worried. If the movie stays in production hell, we'll know. If it is made and bad, we'll know. If the movie is made and good, we'll know.