Tell that to the DC Animated Universe, where John Stewart was THE Green Lantern, save for a couple cameos from Hal Jordon and Kyle Rainer. Part of the reason he's as popular and well known as he is now was he was featured so prominently.
Plus, if you really want a Justice League movie, Stewart makes a much better fit than Jordon does. Jordon is a brash, bold flyboy with a quick with and snarky tongue - in otherwords, a role that in the Justice League would already be taken by the Flash. Stewart, being a military veteran, would serve as the team strategist and NCO, as well as the Straight Man to the Flash's comedic mannerisms, and serving as the team NCO would make him the go to second in command for whether Superman, Batman or Aquaman takes the helm of the Justice League.
Plus, from a marketing and press standpoint - he would be the first prominent, memorable and well developed black superhero, and seeing him stand shoulder to shoulder with Superman or Batman would both generate serious buzz and attract butts to the seats.
I was thinking more
Battle Los Angeles than
Transformers, but yeah, you get the idea. Hell, for extra credit, make them the DC War Comics heroes, allowing a possible film based off of
Men of War or
GI Combat - you give me a movie that has John Stewart AND Frank Rock, I'll see it five times myself