Sorry, but John Stewart did bring something to the table to balance out the JLU. Wasn't just the need of a black character.
I love Kyle over John Stewart by a long shot, but at least in the JLU dynamic, the Flash character and the Kyle character would have been redundant.
Flash works best as the wise-cracking hotshot and you only need one of those. John brought a honorable everyman perspective to the table.
If they ever did a GL movie, I want Kyle. But in the JL dynamic... John actually does work better.