1. Franchise, Baby, Franchise!
This reason is aimed squarely at the good folks at Warner Brothers. The idea of a Batman movie that features a ton of Batman characters leaves the door open for any number of sequels, prequels, and spin-offs. In the story Batman alludes to a lot of his history, notably with Jason Todd, meaning we get a big slice of Batman lore in one go. And rather excitingly, once you set this up you can do an entire Death In The Family movie that chronicles the history of all the Robins, from Dick Grayson to Tim Drake. And you could call it Batman & Robin... wait, dont do that. And you could do the story of how Barbara Gordon became Oracle. If you set up these characters once, they can show up again and again leaving a completely open landscape of films that cover the Bat Universe (yes, okay, I guess you could do The Dark Knight Returns at some point). The fan service alone would be too good to miss - how awesome would it be if the first movie ends with a Superman showing up ready to fight Batman cliffhanger? Printing money people, printing money. I guess what Im trying to say is go read Batman: Hush. If I missed anything let me know in the comments.