1. Avengers only had 3 superheroes lead in to it. Man of Steel should it lead in to JL would be one. We have already seen batman to death. Green Lantern was just aired, even though it may or may nor lead in to JL, the public already got a taste of the character. Despite the average reviews and fanboy rage, the general public that watched it didnt hate it. So thats 3 characters already. Yes having lead in films would let people care more about the characters. But you can't deny the fact that prior to Iron man and Thor, almost no one bothered about both characters. They were never making any headlines in pop culture or had any ground breaking stuff going on. Hulk is an iconic character and Captain America kinda does symbolises america a little, but for years, Superman has been in that primary role. I watched a talk show recently who showed clips of several presidential candidate on who their favourite Superhero is, everyone said Superman except Ron Paul who declined to answer. The commenters even say that Superman is kinda the go-to answer for favourite Superhero because of his symbolism.
My point being, Marvel has got to have all these lead in films because those characters are not iconic characters. But seriously, how many times have we heard the Superman vs Batman conversation? Wonder Woman is the first female superhero and is very symbolic to women. Despite watching a solo WW movie or not, people would care about WW because of what she symbolises. So Justice League do not have to lead in as long as a good script is written.
2. Yes yes yes, but how much of the film would be focusing on Batman? The Hulk recently had 2 films both with different actor. Edward Norton did a great job being the Hulk, even the animated series tried to model after him. But they still replaced him. Sure you can't compare it to Batman, but how often is Batman going to be out of his mask? How much screen time would he actually get? Everyone knows the trilogy ends Christian Bale's take on Batman. But this film really isn't another Batman origins. Its a totally different movie.
3. Yes he is the main character, but the show is for a different premise. He would be almost as important as 4-6 other characters.
Again, having said all these, I'm just saying there isn't a need to wait ten years to have all the characters have a solo movie to lead in to Justice League. Whether or not we would get a Justice League movie is another thing. If we do get a movie, I'm 100% on board with only Man of Steel leading into the movie. If we don't get a movie, then all these are just pointless conversation.