First off, once they gather in the JLA movie and everyone goes to see it, seperating a character that the world barely knows (like Green Lantern, who is still quite unknown in mainstream USA) would seem to be a disappointment to people.
A disappointment on what level?
I have aways thought (and still believe) that Wolverine will gross far less than the average X Men movie.
Of course it will. WOLVERINE, unlike GREEN LANTERN and THE FLASH, will also cost less to make than the other, more recent ensemble films. Therefore, it will be successful, and will be successful having spun of the X-Men franchise.
Its a theory, but a solid one. Why get excited about a solo adventure when we just saw a massive crossover dream team???
Because the individual characters have value.
Secondly, it adds weight to the film.
This assumes people will have seen the solo films coming into a Justice League one.
Lets say you have a Flash film where Barry is established, and maybe have a successor waiting in the wings. If Barry Allen dies early in the JLA movie, then his successor would have already been introduced, BUT the death of Barry Allen would actually have weight to it since people had already invested 2 hours into the character.
Without an introduction he is just some nobody who gets killed off quick. Who among us would want that for Barry?
What makes you think they won't introduce Barry Allen in JUSTICE LEAGUE? The opening is clearly meant to be a "Who dies" moment for casual fans. Yes, FANS know it's likely Barry's funeral, but the average person doesn't. Therefore, when JUSTICE LEAGUE builds Barry as a likeable character, and then kills him, his death will have the weight you speak of.
Have a Wonder Woman film and the already green lit Green Arrow film (They really should put Green Arrow in the freakin JLA movie). Batman and Supes will have a few movies under their belts. Even if Flash or Green Arrow underperforms at the box office, it adds a bit of weight to a group movie.
Which brings me to: If JUSTICE LEAGUE can't maintain "weight" on its own, it's screwed anyway. There shouldn't need to be solo efforts to provide "weight" to these characters and their lives.
I dont think you need ALL of the characters to have solo films. Martian Manhunter does not deserve one...
He most certainly does.
BUT you could introduce him at the beginning of JLA and make him the voice of the movie, which would be interesting since people would be expecting Supes or Batman to be the "stars".
Which seems to be why they're centering things around The FLash.
Third, by waiting a year or so it will enable Batman in his Nolanverse to actually reach a level where his character is seasoned and would be able to do what is called for in the JLA film.
Nolan's Batman is already seasoned and very capable. But he will likely never resemble the man who builds a satellite and spies on his friends. Nolan's Batman will always be "early in his career".