Batman and Superman, no, I don't think necessarily need introduction movies. They're pretty heavily ingrained in the public consciousness to know who they are, even the modern "darker" version of Batman is known over the Adam West campy version.
Wonder Woman I'd argue would need a movie to erase the 80s TV series version of her to bring audiences "up to date" on what her character is. And as mentioned before Aquaman certainly needs to reprogram people about him. Hell his OWN COMIC in the New 52 needed to do that!
So, clearly, people were able to follow the movie without having to have seen the others. Certainly you were rewarded if you have, but it wasn't a requirement.
Sure they were able to follow it, but seeing the other movies certainly would have increased the enjoyment. (I point out again that 90% of Robert Downey, Jr. cannot be enjoyed in Avengers without seeing, at least Iron Man 1.) It's what I recommended to people when Avengers was on the horizon, I said you don't need to see the other movies but seeing them will certainly make The Avengers more enjoyable as you'll have a better grasp on the characters. The only movie I didn't recommend was The Incredible Hulk as I didn't think it was necessary if not from the casting change alone. (Also because Hulk was probably the only character in The Avengers to have the "Superman/Batman" effect of people knowing who he is just from name alone, a giant green rage monster who comes from David Banner.)
It's not necessary, no, but I wouldn't say it's something that can be completely dismissed either. Again, individual movies worked for all of the Avengers' primary team members, built up the hype over several years and resulted in a billions-of-dollars franchise. Why try and to re-invent the wheel and hope you get lucky with that billion dollars just by tossing the team movie out there right away?
Build up the world, that's what Marvel did. I highly doubt The Avengers would have been as successful if they didn't do this world-building route and built up hype for the movie and anticipation for it over the course four or five years.