Originally Posted by theShape
It really should be called World's Finest because that's what batman and Superman are... two of the finest men this world will see. Similar to how the NYPD are referred to as "New York's finest", World's Finest is a perfect title that aptly describes the two core characters.
While I won't claim to know what the film will be called, I do believe that Batman vs Superman is simply a working title, if that. It's a simple way for the filmmakers to refer to this as-yet-untitled film when describing it.
I'll put it like this:
As a comic book fan I can see what people want it to be called World's Finest.
As a filmmaker I can see why they are reluctant to use World's Finest.