Besides, look at the MCU, all of the characters got "origin movies" before the team movie to establish them. Rush right into a JL movie and people might be afraid that it'll connect too much to Batman movies they've probably not seen, that the characters without movies aren't familiar enough to connect to them (this is a problem I think Hawkeye suffers from in the Avengers. He isn't established.)
Some would feel that way, but I doubt the rank and file of general-audience moviegoers who are the primary target here would know the difference or care. I mean, when Smallville
was on the air, there were a lot of viewers who had no idea
that it had anything to do with Superman. We genre fans are too prone to forget that most people aren't as genre-savvy as we are.
Besides, whether or not you "need" origin movies for these characters or not is beside the point. Aquaman and Wonder Woman are strong characters who DESERVE their own movie.
Of course they are. I just don't agree with the assumption that those movies have
to come before a JL movie. Marvel's already done it that way; if DC just copies it, it'll seem like a copy. Let's at least keep our minds open to the possibility of a different approach.